Matteo Berrettini, of Italy, returns against Novak Djokovic, of Serbia, during the quarterfinals of the U.S. Open tennis tournament, Wednesday, Sept. 8, 2021, in New York. (AP Photo/Frank Franklin II)

Novak Djokovic, of Serbia, prepares to hit a backhand to Matteo Berrettini, of Italy, during the quarterfinals of the U.S. Open tennis tournament Wednesday, Sept. 8, 2021, in New York. (AP Photo/Frank Franklin II)

Novak Djokovic, of Serbia, returns to Matteo Berrettini, of Italy, during the quarterfinals of the U.S. Open tennis tournament Wednesday, Sept. 8, 2021, in New York. (AP Photo/Frank Franklin II)

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)