Novak Djokovic, of Serbia, reacts after scoring a point against Matteo Berrettini, of Italy, during the quarterfinals of the U.S. Open tennis tournament early Thursday, Sept. 9, 2021, in New York. (AP Photo/Frank Franklin II)

Alexander Zverev, of Germany, throws game balls to the crowd after defeating Lloyd Harris, of South Africa, during the quarterfinals of the US Open tennis championships, Wednesday, Sept. 8, 2021, in New York. (AP Photo/Elise Amendola)