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 holds his second place trophy after losing to Alexander Zverev of Germany 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)

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)

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, 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 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)