Novak Djokovic, of Serbia, plays Kei Nishikori, of Japan, during the quarterfinals of the tennis competition at the 2020 Summer Olympics, Thursday, July 29, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Novak Djokovic, of Serbia, plays Kei Nishikori, of Japan, during the quarterfinals of the tennis competition at the 2020 Summer Olympics, Thursday, July 29, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Novak Djokovic, of Serbia, reacts while playing Kei Nishikori, of Japan, during the quarterfinals of the tennis competition at the 2020 Summer Olympics, Thursday, July 29, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Novak Djokovic, of Serbia, plays Kei Nishikori, of Japan, during the quarterfinals of the tennis competition at the 2020 Summer Olympics, Thursday, July 29, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Kei Nishikori, of Japan, plays Novak Djokovic, of Serbia, during the quarterfinals of the tennis competition at the 2020 Summer Olympics, Thursday, July 29, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Novak Djokovic, of Serbia, plays Kei Nishikori, of Japan, during the quarterfinals of the tennis competition at the 2020 Summer Olympics, Thursday, July 29, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Kei Nishikori, of Japan, reacts while playing Novak Djokovic, of Serbia, during the quarterfinals of the tennis competition at the 2020 Summer Olympics, Thursday, July 29, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Novak Djokovic, of Serbia, reacts after winning the first set against Kei Nishikori, of Japan, during the quarterfinals of the tennis competition at the 2020 Summer Olympics, Thursday, July 29, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Kei Nishikori, of Japan, bottom, plays Novak Djokovic, of Serbia, during the quarterfinals of the tennis competition at the 2020 Summer Olympics, Thursday, July 29, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Kei Nishikori, of Japan, plays Novak Djokovic, of Serbia, during the quarterfinals of the tennis competition at the 2020 Summer Olympics, Thursday, July 29, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Kei Nishikori, of Japan, plays Novak Djokovic, of Serbia, during the quarterfinals of the tennis competition at the 2020 Summer Olympics, Thursday, July 29, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Novak Djokovic, of Serbia, plays Kei Nishikori, of Japan, during the quarterfinals of the tennis competition at the 2020 Summer Olympics, Thursday, July 29, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Novak Djokovic, of Serbia, celebrates after defeating Kei Nishikori, of Japan, during the quarterfinals of the tennis competition at the 2020 Summer Olympics, Thursday, July 29, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Kei Nishikori, of Japan, rests between points while playing Novak Djokovic, of Serbia, during the quarterfinals of the tennis competition at the 2020 Summer Olympics, Thursday, July 29, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Novak Djokovic, of Serbia, celebrates after defeating Kei Nishikori, of Japan, during the quarterfinals of the tennis competition at the 2020 Summer Olympics, Thursday, July 29, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Novak Djokovic, of Serbia, plays Kei Nishikori, of Japan, during the quarterfinals of the tennis competition at the 2020 Summer Olympics, Thursday, July 29, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Kei Nishikori, of Japan, reacts while playing Novak Djokovic, of Serbia, during the quarterfinals of the tennis competition at the 2020 Summer Olympics, Thursday, July 29, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Kei Nishikori, of Japan, rests between points while playing Novak Djokovic, of Serbia, during the quarterfinals of the tennis competition at the 2020 Summer Olympics, Thursday, July 29, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Novak Djokovic, of Serbia, celebrates after defeating Kei Nishikori, of Japan, during the quarterfinals of the tennis competition at the 2020 Summer Olympics, Thursday, July 29, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Novak Djokovic, of Serbia, plays Kei Nishikori, of Japan, during the quarterfinals of the tennis competition at the 2020 Summer Olympics, Thursday, July 29, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Novak Djokovic, of Serbia, returns to Alexander Zverev, of Germany, during the semifinal round of the men's tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Patrick Semansky)

Novak Djokovic, of Serbia, returns to Alexander Zverev, of Germany, during the semifinal round of the men's tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Patrick Semansky)

Alexander Zverev, of Germany, returns to Novak Djokovic, of Serbia, during the semifinal round of the men's tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Patrick Semansky)

Novak Djokovic, of Serbia, serves to Alexander Zverev, of Germany, during the semifinal round of the men's tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Patrick Semansky)

Novak Djokovic, of Serbia, returns to Alexander Zverev, of Germany, during the semifinal round of the men's tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Patrick Semansky)

Novak Djokovic, of Serbia, returns to Alexander Zverev, of Germany, during the semifinal round of the men's tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Patrick Semansky)

Alexander Zverev, of Germany, left, hugs Novak Djokovic, of Serbia, after defeating him during a semifinals match of the tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Alexander Zverev, of Germany, reacts after defeating Novak Djokovic, of Serbia, during a semifinals match of the tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Alexander Zverev, of Germany, reacts after defeating Novak Djokovic, of Serbia, during a semifinals match of the tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Alexander Zverev, of Germany, left, hugs Novak Djokovic, of Serbia, after defeating him during a semifinals match of the tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Alexander Zverev, of Germany, serves to Novak Djokovic, of Serbia, during the semifinal round of the men's tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Patrick Semansky)

Alexander Zverev, of Germany, reacts during a semifinal men's tennis match against Novak Djokovic, of Serbia, at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Patrick Semansky)

Novak Djokovic, of Serbia, reacts during a semifinal men's tennis match against Alexander Zverev, of Germany, at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Patrick Semansky)

Alexander Zverev, of Germany, reacts after defeating Novak Djokovic, of Serbia, during a semifinals match of the tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Alexander Zverev, of Germany, reacts after defeating Novak Djokovic, of Serbia, during a semifinals match of the tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Alexander Zverev, left, of Germany, speaks with Novak Djokovic, of Serbia, after defeating Djokovic in the semifinal round of the men's tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Patrick Semansky)

Alexander Zverev, of Germany, returns to Novak Djokovic, of Serbia, during the semifinal round of the men's tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Patrick Semansky)

Novak Djokovic, of Serbia, serves to Alexander Zverev, of Germany, during the semifinal round of the men's tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Patrick Semansky)

Alexander Zverev, of Germany, returns to Novak Djokovic, of Serbia, during the semifinal round of the men's tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Patrick Semansky)

Alexander Zverev, of Germany, serves to Novak Djokovic, of Serbia, during the semifinal round of the men's tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Patrick Semansky)

Novak Djokovic, of Serbia, serves to Alexander Zverev, of Germany, during the semifinal round of the men's tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Patrick Semansky)

Novak Djokovic, bottom right, of Serbia, serves to Alexander Zverev, of Germany, during the semifinal round of the men's tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Patrick Semansky)

Novak Djokovic, of Serbia, reacts after being defeated by Alexander Zverev, of Germany, during a semifinals match of the tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Novak Djokovic, of Serbia, reacts during a semifinal men's tennis match against Alexander Zverev, of Germany, at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Patrick Semansky)

Novak Djokovic, of Serbia, reacts during a semifinal men's tennis match against Alexander Zverev, of Germany, at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Patrick Semansky)

Novak Djokovic, of Serbia, wipes sweat from his forehead during a semifinal men's tennis match against Alexander Zverev, of Germany, at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Patrick Semansky)

Alexander Zverev, of Germany, reacts after defeating Novak Djokovic, of Serbia, during the semifinal round of the men's tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Patrick Semansky)

Alexander Zverev, of Germany, reacts after defeating Novak Djokovic, of Serbia, during the semifinal round of the men's tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Patrick Semansky)

Alexander Zverev, left, of Germany, speaks with Novak Djokovic, of Serbia, after defeating Djokovic in the semifinal round of the men's tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Patrick Semansky)

Alexander Zverev, of Germany, reacts after defeating Novak Djokovic, of Serbia, during a semifinals match of the tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Alexander Zverev, of Germany, left, hugs Novak Djokovic, of Serbia, after defeating him during a semifinals match of the tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Alexander Zverev, of Germany, plays Novak Djokovic, of Serbia, during a semifinals match of the tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Novak Djokovic, of Serbia, reacts during a semifinal men's tennis match against Alexander Zverev, of Germany, at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Patrick Semansky)

Novak Djokovic, of Serbia, reacts after being defeated by Alexander Zverev, of Germany, during a semifinals match of the tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Alexander Zverev, of Germany, reacts after defeating Novak Djokovic, of Serbia, during a semifinals match of the tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Novak Djokovic, of Serbia, reacts after being defeated by Alexander Zverev, of Germany, during a semifinals match of the tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

Novak Djokovic, of Serbia, walks off the court after losing a semifinal men's tennis match to Alexander Zverev, of Germany, at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Patrick Semansky)

Alexander Zverev, of Germany, embraces Novak Djokovic, of Serbia, after defeating Djokovic in the semifinal round of the men's tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Patrick Semansky)

Alexander Zverev, of Germany, returns to Novak Djokovic, of Serbia, during the semifinal round of the men's tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Patrick Semansky)

Alexander Zverev, of Germany, reacts after defeating Novak Djokovic, of Serbia, during the semifinal round of the men's tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Patrick Semansky)

Alexander Zverev, of Germany, reacts after defeating Novak Djokovic, of Serbia, during the semifinal round of the men's tennis competition at the 2020 Summer Olympics, Friday, July 30, 2021, in Tokyo, Japan. (AP Photo/Patrick Semansky)

Novak Djokovic, of Serbia, returns a shot to Alexander Zverev, of Germany, during the bronze medal match of the tennis competition at the 2020 Summer Olympics, Saturday, July 31, 2021, in Tokyo, Japan. (AP Photo/Seth Wenig)

FILE - In this July 28, 2021, file photo, Novak Djokovic, of Serbia, returns to Alejandro Davidovich Fokina, of Spain, during the third round of men's tennis at the 2020 Summer Olympics in Tokyo. If Djokovic wins the U.S. Open to complete the first calendar-year Grand Slam in men's tennis in 52 years, "Rocket" Rod Laver will be there to welcome him into a remarkably exclusive club. (AP Photo/Patrick Semansky, File)