Defending champion Aryna Sabalenka advanced to the U.S. Open semifinals on Tuesday without playing after Marketa Vondrousova withdrew from their quarterfinal with an injury.

The U.S. Tennis Association announced the walkover roughly two hours before the match was due to start in Arthur Ashe Stadium and did not specify the nature of Vondrousova’s problem. The 26-year-old left-hander had reached the quarterfinals by beating 2022 Wimbledon champion Elena Rybakina in three sets on Sunday night.

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

Vondrousova has reached as high as No. 6 in the WTA rankings but has missed stretches of the tour because of injuries, including operations on her wrist in 2022 and on her shoulder last year. She entered the U.S. Open ranked 60th and unseeded.

Pegula reached her second Grand Slam semifinal by defeating two-time major champion Barbora Krejcikova 6-3, 6-3 earlier Tuesday. Pegula had been 0-6 in major quarterfinals until upsetting Iga Swiatek 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 U.S. Open in consecutive years without losing a set since 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 remaining two women’s quarterfinals are scheduled for Wednesday: No. 2 Iga Swiatek vs. No. 8 Amanda Anisimova, a rematch of last month’s Wimbledon final that Swiatek won 6-0, 6-0, and No. 11 Karolina Muchova vs. No. 23 Naomi Osaka.

This US Open delivered an unusually heavy load of five-setters, and the consequences were plain. Three players, Flavio Cobolli, Kamil Majchrzak, and Daniel Altmaier, retired on Saturday after winning marathons on Thursday. Tommy Paul seemed to run out of gas after playing his second wee-hour five-setter in a row. The player who beat him, Alexander Bublik, then experienced a similar collapse against Jannik Sinner.

The long-running argument over whether men should keep best-of-five at the majors continues. As one observer put it when hearing best-of-five called “the ultimate test in tennis,” the response is often, “So why don’t the women get to take the same test?”

Still, many regard five-set Slams as sacrosanct. They have produced epic, defining moments and have not, historically, shortened careers or led to an obvious rash of retirements. Yet the modern game is more physical, equipment is more advanced, and prolonged baseline warfare can turn best-of-five into four-hour battles of attrition. Even winners can be so spent that they are compromised for the next match.

One proposal to ease the load while preserving the format is to adopt no-ad scoring. Eliminating deuce games caps the maximum points in a game at seven and thus limits the maximum number of points in a set. Shorter matches mean less cumulative wear and tear. The strategy and winner-take-all aspect of the no-ad point would add another element of suspense to matches and could make long fifth sets easier for fans to watch.

The Roland Garros final between Sinner and Carlos Alcaraz underlines the issue: they played 56 games and 352 points in those games, plus 33 more points in three tiebreakers. They played “at least five deuce games,” and the first game went to five deuces. No-ad would have made that final shorter, though by how much is a question worth answering.

No-ad is not new to the sport. The author played it in high school and college in the early 1990s, and the college game has more recently returned to no-ad. Change in tennis often needs a champion and a pathway through junior and lower-level events to build acceptance. The question is whether no-ad could be that pathway to protect players while keeping five-set drama intact.

Two months after a one-sided Wimbledon final that finished 6-0, 6-0, Amanda Anisimova and Iga Swiatek meet again in the US Open quarterfinals. The rematch pairs a 24-year-old American who has been open about mental health struggles and a hiatus from the tour with a six-time Grand Slam champion from Poland.

Anisimova, ranked No. 8 and a one-time major finalist, advanced to the last eight with a win over Beatriz Haddad Maia. “I’m really excited and looking forward to it,” Anisimova said after booking her place in the quarters. “At this stage of the game you’re going to play a really tough opponent, regardless. So, to be able to have a rematch, to face her again and give myself another chance, I’m really, really happy about that…It’s going to be a really tough challenge, but I feel like I’ve been playing well.”

Swiatek, ranked No. 2, declined to comment on the pairing immediately after her fourth-round win. “There’s no point now for me to overthink who I’m going to play. I’m just going to see who wins, and that’s it. I’ll prepare.”

Beyond styles and statistics, this match carries an emotional subtext. The Wimbledon champion will aim to control points with heavy first serves and topspin forehands that push Anisimova back and deny her time to set up penetrating groundstrokes. Anisimova has worked to improve her serve to seize initiative and blunt Swiatek’s aggressive returns.

Neither player holds a glaring statistical edge, which underlines how a handful of points can decide a match. Anisimova has acknowledged the pain of the final at Wimbledon and resisted the notion that it made her a better player. “I don’t think it (losing) helped me become a better player in any way,” she said, explaining that “bouncing back” over the summer was somewhat difficult because that Wimbledon situation was so novel. “It wasn’t a good performance by any means. I feel like maybe I learned some things from it, and some things I can do differently. But above all, I think it was just a learning experience.”

Anisimova, who won the junior singles title here, arrives as a sentimental favorite. Swiatek has repeatedly said the US Open presents different pressures. “For sure it’s harder to focus here (than at Wimbledon),” she said. On the question of relentless play after Wimbledon, Swiatek added: “I think most of us were raised that way, to win every point. But obviously, because of the score at Wimbledon, everybody started asking this question. When you don’t play sports, you don’t get it, but if you do, then you know that you’re not going to give anything for free.”