Defending champion Aryna Sabalenka returned to the U.S. Open semifinals without hitting a ball on Tuesday night because the woman she was supposed to face in the quarterfinals, 2023 Wimbledon champion Marketa Vondrousova, pulled out of their match with an injured knee.

According to the U.S. Tennis Association, Vondrousova is the first woman to give her opponent a walkover — the term for when a tennis player withdraws from a match before it begins — at the U.S. Open in the quarterfinals or later since 1988, when Steffi Graf advanced to the final when Chris Evert did not play their semifinal.

“I tried my best to take the court today, but during the warm-up, I felt again my knee, and after consultation with the tournament doctor decided not to risk aggravating the injury," Vondrousova, a 26-year-old left-hander from the Czech Republic, said in a statement released by the USTA. “I appreciate all the support this tournament and apologize to the fans who were looking forward to the match.”

She had advanced to the quarterfinals with a three-set victory over 2022 Wimbledon champion Elena Rybakina on Sunday night.

With one of the two nighttime matches in Ashe scrapped Tuesday, the USTA didn't replace Sabalenka vs. Vondrousova on the program with another contest, but set up the men's quarterfinal between 24-time major champion Novak Djokovic and 2024 runner-up Taylor Fritz to begin at 8 p.m. That was originally supposed to start whenever Sabalenka-Vondrousova ended.

Sabalenka now will meet No. 4 Jessica Pegula in the semifinals on Thursday. That is a rematch of last year’s championship match at Flushing Meadows, which Sabalenka won in straight sets.

Vondrousova has been as high as No. 6 in the WTA rankings but a series of injuries over the years kept off the tour for stretches at a time. That included operations on her wrist in 2022 and on her shoulder last year.

She entered the U.S. Open ranked 60th and unseeded.

“So sorry for Marketa after all she’s been through,” Sabalenka wrote on social media. “She has been playing amazing tennis and I know how badly this must hurt for her.”

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