New York — Jessica Pegula pulled off a major upset of Iga Swiatek at the U.S. Open, beating the No. 1-ranked woman 6-2, 6-4 on Wednesday night to win a Grand Slam quarterfinal for the first time on her seventh try.

“Finally, I can say ‘semifinalist!’” Pegula told the crowd.

“I knew I could do it,” she added.

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.

The No. 6-seeded Pegula, a 30-year-old American, has won 14 of her past 15 matches and will make her debut appearance in the semifinals of a Slam on Thursday against unseeded Karolina Muchova of the Czech Republic.

Pegula repeatedly did what seemed nearly impossible to do lately 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: break her serve.

Entering Wednesday, Swiatek had lost just a pair of service games across four matches in the hard-court 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% — in play at the start, only 36% for the opening set.

During Pegula’s previous 0-6 rut in major quarterfinals, two of the exits came against Swiatek, and one was against another No. 1 player, Ash Barty.

Pegula was asked about that record in this round during her post-match interview on court after winning her fourth-round match. And again in the news conference that followed. And again during a TV interview before stepping out into Arthur Ashe Stadium on Wednesday.

If that bothered her, Pegula didn’t let it show. Much the way she didn’t show any perceptible emotion after going up 4-0 just 21 minutes into the quarterfinal. Even when Pegula grabbed that set, there was just a slight shake of her left fist as she walked to her sideline seat.

Swiatek didn’t hide her frustration that well, including a big slap of 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.

Muchova was the runner-up to Swiatek at the 2023 French Open and made it to the final four at Flushing Meadows for the second consecutive year with a 6-1, 6-4 victory over No. 22 Beatriz Haddad Maia earlier Wednesday.

“It was me and myself, it was my ghosts inside my mind and I know all the tennis players have that,” Haddad Maia said. “Today was like an inner fight. I couldn’t manage that.”

Muchova lost to champion Coco Gauff in the U.S. Open semifinals in 2023, but then needed surgery on her right wrist in October and was off the tour for about 10 months, 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.”

“Now, looking back,” she said, “I’m, like, ‘Oh, it actually flew by, the time, and I feel strong again.’”

The other women’s singles match Thursday will be Australian Open champion Aryna Sabalenka, who is seeded No. 2, against first-time major semifinalist Emma Navarro, an American seeded 13th. Sabalenka lost to Gauff in the 2023 final in New York; Navarro ended Gauff’s title defense in the fourth round.

In the men’s quarterfinals, No. 25 Jack Draper reached his first Grand Slam semifinal — and became the first British man to get that far at the U.S. Open since Andy Murray won the 2012 trophy — by overwhelming No. 10 Alex de Minaur 6-3, 7-5, 6-2.

The other men’s semifinal will be No. 12 Taylor Fritz vs. No. 20 Frances Tiafoe in the first all-American matchup at this stage at a major in 19 years.

Draper has won all 15 sets he’s played so far, but things figure to get tougher on Friday: That’s when he will take on either No. 1 Jannik Sinner or No. 5 Daniil Medvedev for a berth in the final. Medvedev won the title in New York in 2021 and was the only past champion in the men’s bracket heading into his showdown with Sinner in the last singles quarterfinal on Wednesday.

“This is not kind of like an overnight thing for me. I’ve believed for a long time that I’ve been putting in the work and doing the right things, and I knew that my time would come,” said Draper, whose upper right leg was taped by a trainer after he felt something at the end of the first set. “I didn’t know when it would be, but hopefully from here, I can do a lot of amazing things. I’m very proud of myself.”