American Jessica Pegula beats two-time Wimbledon champion Petra Kvitova to advance to the US Open quarterfinals. (0:28)

NEW YORK -- Iga Swiatek covered her head with a white towel during one changeover after falling behind by a set and a break in the US Open's fourth round Monday. She kept making mistakes, then rolling her eyes or glaring in the direction of her guest box.

Eventually, Swiatek got her strokes straightened out and began playing more like someone who is ranked No. 1, moving into her first quarterfinal at Flushing Meadows by coming back to beat Jule Niemeier 2-6, 6-4, 6-0 in Louis Armstrong Stadium.

"I'm just proud," Swiatek said, "that I didn't lose hope."

The 21-year-old from Poland will face another first-time US Open quarterfinalist next. That's No. 8 seed Jessica Pegula, the highest-ranked American woman, who advanced earlier Monday with a 6-3, 6-2 victory over two-time Wimbledon winner Petra Kvitova in a match interrupted by rain for 45 minutes during the third game because the retractable roof at Arthur Ashe Stadium was not closed when a drizzle became a downpour.

Pegula called it "a little frustrating," saying it was particularly tough "at the start of the match, when there's no feeling of what's going on. You're just sitting there in limbo, not sure what to do."

The U.S. Tennis Association said in a statement that its "weather team" told US Open organizers there was no rain expected. The statement added: "Unfortunately, a pop-up sunshower occurred."

Later Monday, two-time Australian Open champion Victoria Azarenka lost 7-5, 6-7 (5), 6-2 to two-time major finalist Karolina Pliskova, who advances to face No. 6 Aryna Sabalenka. Pliskova, a former world No. 1, was the runner-up at the US Open in 2016 and at Wimbledon in 2021.

Pliskova has returned to form in Flushing Meadows after a broken right wrist forced her to miss the Australian Open and she struggled in her first few tournaments when she returned to action.

Azarenka lost for the fifth time in nine career meetings against Pliskova.

In the night session, Sabalenka bounced back from a slow start and visits from the trainer to outlast No. 19 seed Danielle Collins 3-6, 6-3, 6-2.

Sabalenka continued her mastery at Flushing Meadows over Collins, beating her for the third time at the US Open (2018 and 2021). She led 4-3 in the second set when she needed a medical timeout. Sabalenka had a trainer massage her left thigh, and it continued throughout breaks in the match. Sabalenka, an Open semifinalist in 2021, said at her on-court interview she was fine.

Sabalenka at one point complained to the chair umpire about noise from the fans -- and again, there were plenty of them inside Arthur Ashe Stadium. The U.S. Open has attracted a record 549,657 fans since the main draw started last Monday. It's the highest attendance for the tournament through its first eight days. The previous record was 540,333, set in 2019.

She even changed her racket -- which she said was "super loose" -- in the midst of the second set. It didn't bother her too much. She had 38 winners.

Sabalenka, who is from Belarus, was not allowed to play at Wimbledon this year after all players from her country and Russia were banned because of the invasion of Ukraine.

Pegula, a 28-year-old who was born in New York and whose parents own the NFL's Buffalo Bills and NHL's Buffalo Sabres, reeled off the last three games of Monday's opening set, which ended with a double fault by No. 21 Kvitova, and then the last six games after trailing 2-0 in the second.

This will be Pegula's third appearance in a major quarterfinal this season -- and third against the woman who led the rankings at the time. She lost to the since-retired Ash Barty at the Australian Open, which Barty ended up winning, then to Swiatek at the French Open, which Swiatek wound up winning for her second championship there.

"I just have to play within myself," Pegula said, "and play smart and be present."

Swiatek arrived in New York having gone just 4-4 since the end of a 37-match winning streak that carried her to six consecutive tournament titles. On Monday, she listened to her usual prematch playlist -- a mix of Pearl Jam, AC/DC, Led Zeppelin and Gorillaz -- then came out flat as can be against Niemeier, who is ranked 108th but reached the quarterfinals at Wimbledon in July in just her second major tournament.

Niemeier was better in the first set. Far better. Swiatek produced only four winners and 13 unforced errors, got broken twice and never earned a single break point.

Then, just winning the second set to pull even was quite the struggle for Swiatek.

She got broken early and fell behind 2-1, which is when she draped herself with a towel. She seemed to be talking to herself under there.

Whatever went on, Swiatek immediately flipped things around with a three-game run to lead 4-2. Even after getting a chance to serve for that set at 5-3, she double-faulted to close that game.

Nothing, it seemed, would come easily on this muggy afternoon, with the temperature above 80 degrees Fahrenheit (26.7 Celsius) and the humidity above 50%.

But Swiatek did break right back, helped by a wild forehand from Niemeier, and she let out a shout.

To a third set they went, and Swiatek completely cleaned up her act: The unforced error count was one for her and 14 for Niemeier. When it ended, Swiatek jumped and threw an uppercut and shook her right fist.

The women's field is guaranteed to have a first-time champion.