Alexander Zverev of Germany returns with a backhand to Daniil Medvedev of Russia during their singles final tennis match of the ATP World Tour Finals, at the Pala Alpitour in Turin, Italy, Sunday, Nov. 21, 2021. (AP Photo/Luca Bruno)

France's Pierre-Hugues Herbert and Nicolas Mahut, left, pose with their trophy after winning the doubles final tennis match of the ATP World Tour Finals, at the Pala Alpitour in Turin, Italy, Sunday, Nov. 21, 2021. Herbert and Mahut defeated Rajeev Ram of the United States and Britain's Joe Salisbury 6-4/7-6. (AP Photo/Luca Bruno)

Alexander Zverev of Germany, with his brother Misha, left, holds his trophy after winning the singles final tennis match of the ATP World Tour Finals, at the Pala Alpitour in Turin, Italy, Sunday, Nov. 21, 2021. Zverev defeated Daniil Medvedev of Russia 6-4/6-4.(AP Photo/Luca Bruno)

Alexander Zverev of Germany reacts after defeating Daniil Medvedev of Russia in their singles final tennis match of the ATP World Tour Finals, at the Pala Alpitour in Turin, Italy, Sunday, Nov. 21, 2021. Zverev won 6-4/6-4.(AP Photo/Luca Bruno)

Daniil Medvedev of Russia gestures during his singles final tennis match of the ATP World Tour Finals against Alexander Zverev of Germany, at the Pala Alpitour in Turin, Italy, Sunday, Nov. 21, 2021. (AP Photo/Luca Bruno)

Alexander Zverev of Germany holds his trophy after winning the singles final tennis match of the ATP World Tour Finals, at the Pala Alpitour in Turin, Italy, Sunday, Nov. 21, 2021. Zverev defeated Daniil Medvedev of Russia 6-4/6-4.(AP Photo/Luca Bruno)

Alexander Zverev of Germany reacts after defeating Daniil Medvedev of Russia in their singles final tennis match of the ATP World Tour Finals, at the Pala Alpitour in Turin, Italy, Sunday, Nov. 21, 2021. Zverev won 6-4/6-4.(AP Photo/Luca Bruno)

Daniil Medvedev of Russia returns the ball to Alexander Zverev of Germany during their singles final tennis match of the ATP World Tour Finals, at the Pala Alpitour in Turin, Italy, Sunday, Nov. 21, 2021. (AP Photo/Luca Bruno)

Alexander Zverev of Germany plays with his dogs after winning the singles final tennis match of the ATP World Tour Finals, at the Pala Alpitour in Turin, Italy, Sunday, Nov. 21, 2021. Zverev defeated Daniil Medvedev of Russia 6-4/6-4.(AP Photo/Luca Bruno)

Alexander Zverev of Germany signs autographs for fans after winning the singles final tennis match of the ATP World Tour Finals, at the Pala Alpitour in Turin, Italy, Sunday, Nov. 21, 2021. Zverev defeated Daniil Medvedev of Russia 6-4/6-4.(AP Photo/Luca Bruno)