Karolina Pliskova, of the Czech Republic, reacts after missing a shot to Maria Sakkari, of Greece, during the quarterfinals of the U.S. Open tennis tournament Wednesday, Sept. 8, 2021, in New York. (AP Photo/Frank Franklin II)

Belinda Bencic, of Switzerland, returns a shot to Emma Raducanu, of Great Britain, during the quarterfinals of the US Open tennis championships, Wednesday, Sept. 8, 2021, in New York. (AP Photo/Elise Amendola)

Alexander Zverev, of Germany, returns a shot to 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)

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)

Emma Raducanu, of Great Britain, returns a shot to Belinda Bencic, of Switzerland, during the quarterfinals of the US Open tennis championships, Wednesday, Sept. 8, 2021, in New York. (AP Photo/Elise Amendola)

Lloyd Harris, of South Africa, returns a shot to Alexander Zverev, of Germany, during the quarterfinals of the US Open tennis championships, Wednesday, Sept. 8, 2021, in New York. (AP Photo/Elise Amendola)

Maria Sakkari, of Greece, reacts after winning the first set against Karolina Pliskova, of the Czech Republic, during the quarterfinals of the U.S. Open tennis tournament Wednesday, Sept. 8, 2021, in New York. (AP Photo/Frank Franklin II)

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)