NEW YORK -- Defending champion Aryna Sabalenka returned to the US 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.
"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 United States Tennis Association. "I appreciate all the support this tournament and apologize to the fans who were looking forward to the match."
A representative for Vondrousova told ESPN's Mary Joe Fernandez the injury occurred before her three-set victory over 2022 Wimbledon champion Elena Ryakina and that Vondrousova played through it during the fourth-round match Sunday night.
According to the USTA, 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 US Open in the quarterfinals or later since 1988, when Steffi Graf advanced to the final after Chris Evert did not play their semifinal.
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.
This is the first time Sabalenka has ever advanced in a major via walkover or retirement.
"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."
Sabalenka told ESPN's Kris Budden she was "shocked" by the news after watching Vondrousova warm up and that she wished Vondrousova a speedy recovery in the locker room.
Vondrousova has been as high as No. 6 in the WTA rankings, but a series of injuries over the years kept her 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.
"This place is so special to me -- exactly one year ago I was in surgery, not even sure if I would ever return," Vondrousova wrote in an Instagram story. "Being back here, competing again and enjoying such beautiful matches has meant more than words can say."
The last two women's quarterfinal matches are scheduled for Wednesday: No. 2 Iga Swiatek vs. No. 8 Amanda Anisimova -- that's a rematch of last month's Wimbledon final, which Swiatek won 6-0, 6-0 -- and No. 11 Karolina Muchova vs. No. 23 Naomi Osaka.
That will be Osaka's first Grand Slam quarterfinal since the 2021 Australian Open, where she wound up claiming her fourth major trophy.
ESPN Research and The Associated Press contributed to this report.