The questions wouldn't stop for Jessica Pegula: Why was she 0-6 in Grand Slam quarterfinals heading into her U.S. Open matchup against No. 1 Iga Swiatek? What could Pegula do to change that?

Came up during her on-court interview after winning in the previous round. And again at the news conference that followed. And again during a brief TV interview right before striding onto the court at Arthur Ashe Stadium on Wednesday night.

If that all weighed on Pegula, the 30-year-old American hid it well, pulling off a big upset by easily beating Swiatek 6-2, 6-4 at Flushing Meadows and earning a debut trip to the semifinals at a major.

"There have been so many freaking times, and I just kept losing," said Pegula, who has won 14 of her past 15 matches, all on hard courts. "I know everyone keeps asking me about it, but I was like, `I don't know what else to do. I just need to get there again and, like, win the match.' So thank God I was able to do it. And finally — finally! — I can say, `Semifinalist."'

She will face unseeded Karolina Muchova of the Czech Republic on Thursday for a berth in the final.

Muchova, the runner-up to Swiatek at the 2023 French Open, made it to the final four in New York for the second consecutive year with a 6-1, 6-4 victory over No. 22 seed Beatriz Haddad Maia earlier Wednesday. Shortly after losing to Coco Gauff in last year's semifinals, Muchova had surgery on her right wrist, then was off the tour for about 10 months until returning this June.

That was the latest in a series of injuries for Muchova, who called it "one of the worst ones that I had."

"Doesn't seem to matter how many matches she's really gotten in; she seems to have good results no matter what. It's cool to kind of see her back, because I think she's really great for the game and the way she plays is really fun," said the No. 6-seeded Pegula, who defeated Muchova at the Cincinnati Open last month. "She's good, so talented, so skilled as a tennis player. Like, so complete. Doesn't have a ton of weaknesses."

The other women's match Thursday night also will feature an American making her major semifinal debut, No. 13 Emma Navarro, against No. 2 Aryna Sabalenka, who won the past two Australian Open titles. Sabalenka lost to Gauff in the 2023 final in New York; Navarro ended Gauff's title defense in the fourth round.

There are two Americans in the men's semifinals, too, and they'll face each other: No. 12 Taylor Fritz takes on No. 20 Frances Tiafoe on Friday.

This is the first time since the 2003 U.S. Open that multiple Americans reached both the women's and the men's semis.

The other men's semifinal will be No. 1 Jannik Sinner against No. 25 Jack Draper. Sinner defeated 2021 champion Daniil Medvedev 6-2, 1-6, 6-1, 6-4 on Wednesday night to reach the final four at Flushing Meadows for the first time. Draper eliminated Alex de Minaur in straight sets.

The lopsided nature of Pegula's win was surprising, but she did not think this day would never arrive.

"I knew I could do it," she said. "I just had to go out and execute my game and not get frustrated."

Swiatek served poorly in the first set and her forehand was a real problem, with 22 of her 41 unforced errors coming on that side. Pegula made only 22 unforced errors in all and used terrific defense to keep forcing Swiatek to hit an extra shot.

"It's never easy to play against Jess. She has a tricky ball because it's pretty low and pretty flat," Swiatek said. "I just made too many mistakes."

Pegula also kept breaking serve against Swiatek, who counts the 2022 U.S. Open among her five Grand Slam titles and has led the WTA rankings for most of the past 2 1/2 years.

Entering Wednesday, Swiatek had lost just a pair of service games across four matches in the tournament, both in the first round — and she didn't even face a single break point in any of her most recent three contests. That's all part of why the 23-year-old from Poland was listed as a -350 money-line favorite against Pegula, according to BetMGM Sportsbook.

But Pegula, whose parents own the NFL's Buffalo Bills and NHL's Buffalo Sabres, didn't have much trouble in that department, especially at the outset, breaking in each of Swiatek's initial two service games, which both ended with double-faults, and three of the first six.

It helped that Swiatek was unable to properly calibrate her first serves early, putting just 2 of 12 — 16.7 per cent — in play at the start, only 36 per cent for the opening set.

"I didn't really understand why my serve wasn't working," Swiatek said.

Even as the games kept going in her favor, Pegula didn't show much perceptible emotion, whether grabbing a 4-0 lead just 21 minutes in or taking up that set, which was greeted with a slight shake of her left fist as she walked to her sideline seat.

Swiatek didn't hide her thoughts that well. She smacked her racket against the top of the net. She slapped her right thigh after a forehand flew wide to get broken yet again and trail 4-3 in the second set.

Fifteen minutes later, it was over.

"I feel like when I have high expectations, I never perform well," Swiatek said. "(But) it's hard to have low expectations when everybody is expecting something from you."