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)