Defending champion Aryna Sabalenka returned to theUS Opensemifinals without hitting a ball on Tuesday night because the woman she was supposed to face in the quarterfinals, 2023 Wimbledon champion Markéta Vondroušová, pulled out of their match with an injured knee. According to the US Tennis Association, Vondroušová 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 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," Vondroušová, 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. Vondroušová 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-Vondroušová 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. Vondroušová 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 US Open ranked 60th and unseeded. "So sorry for Markéta 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." Pegula moved into her second grand slam semifinal by eliminating two-time major champion Barbora Krejčíková 6-3, 6-3 earlier Tuesday. Pegula had been 0-6 in major quarterfinals until upsetting Iga Świątek in that round at Flushing Meadows a year ago. Now the 31-year-old American is the first woman to make the final four at the US Open in consecutive years without losing a set since 23-time major champion Serena Williams did it every year from 2011 to 2014. "I've been able to kind of go into those matches and really take care of business," Pegula said. The last two women's quarterfinals are scheduled for Wednesday: No. 2 Świątek 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 Karolína Muchová 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. For more CNN news and newsletters create an account atCNN.com