Alexander Zverev of Germany reacts after defeating Novak Djokovic of Serbia in their ATP World Tour Finals, singles semifinal, tennis match, at the Pala Alpitour in Turin, Italy, Saturday, Nov. 20, 2021. Zverev won 7-6/4-6/6-3. (AP Photo/Luca Bruno)

Novak Djokovic of Serbia leaves the court after losing to Alexander Zverev of Germany in their ATP World Tour Finals, singles semifinal, tennis match, at the Pala Alpitour in Turin, Italy, Saturday, Nov. 20, 2021. Zverev won 7-6/4-6/6-3. (AP Photo/Luca Bruno)

Daniil Medvedev of Russia reacts after defeating Casper Ruud of Norway in their ATP World Tour Finals, singles semifinal, tennis match, at the Pala Alpitour in Turin, Italy, Saturday, Nov. 20, 2021. Medvedev won 6-4/6-2. (AP Photo/Luca Bruno)

Daniil Medvedev of Russia shake hands with Casper Ruud of Norway, right, at the end of their ATP World Tour Finals, singles semifinal, tennis match, at the Pala Alpitour in Turin, Italy, Saturday, Nov. 20, 2021. Medvedev won 6-4/6-2. (AP Photo/Luca Bruno)

Daniil Medvedev of Russia serves to Casper Ruud of Norway during their ATP World Tour Finals, singles semifinal, tennis match, at the Pala Alpitour in Turin, Italy, Saturday, Nov. 20, 2021. (AP Photo/Luca Bruno)

Daniil Medvedev of Russia hits a forehand to Casper Ruud of Norway during their ATP World Tour Finals, singles semifinal, tennis match, at the Pala Alpitour in Turin, Italy, Saturday, Nov. 20, 2021. (AP Photo/Luca Bruno)

Casper Ruud of Norway returns the ball to Daniil Medvedev of Russia during their ATP World Tour Finals, singles semifinal, tennis match, at the Pala Alpitour in Turin, Italy, Saturday, Nov. 20, 2021. (AP Photo/Luca Bruno)