Jessica Pegula reacts after her victory over Iga Świątek during the quarter-finals of the US Open in New York on Wednesday. Eduardo Munoz Alvarez/Associated Press

NEW YORK — Jessica Pegula caused a major upset at the US Open by defeating Iga Swiatek, the world number one, 6-2, 6-4 on Wednesday night to reach her first Grand Slam quarterfinal in her seventh attempt.

“Finally I can say ‘semi-finalist!’” Pegula told the audience.

“I knew I could do it.”

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 total and used great defense to force Swiatek to hit an extra shot.

Sixth-seeded Pegula, a 30-year-old American, has won 14 of her last 15 matches and makes her Grand Slam semifinal debut on Thursday against unseeded Karolina Muchova.

Pegula repeatedly did what had recently seemed nearly impossible against Swiatek, who counts the 2022 US Open among her five Grand Slam titles and has led the WTA rankings for the past 2½ years: break her serve.

Before Wednesday, Swiatek had lost just two service games in four matches at the hard-court tournament, both in the first round — and she had not faced a break point in any of her last three matches. All of which is why the 23-year-old Pole was listed as a -350 money-line favorite against Pegula, according to BetMGM Sportsbook.

But Pegula, whose parents own the Buffalo Bills (NFL) and Buffalo Sabres (NHL), didn’t have much trouble with that, especially early on. He broke in each of Swiatek’s first two service games, both of which ended with a double fault, and in three of the first six.

It helped that Swiatek couldn’t calibrate her first serves properly early on, putting just 2 of 12 — 16.7% — into play at the start, and just 36% for the opening set.

Pegula’s previous 0-6 run in the quarterfinals of major matches included two eliminations by Swiatek and one by another No. 1, Ash Barty.

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

If that bothered her, Pegula didn’t show it. Just as she showed no discernible emotion after leading 4-0 after just 21 minutes in the quarterfinals.

Even as Pegula took the set, her left fist shook slightly as she walked to her seat on the sideline.

Swiatek couldn’t hide her frustration, including a hard blow to her right thigh after a forehand flew wide and was broken again, and trailed 4-3 in the second set. Fifteen minutes later it was over.

Muchova finished runner-up to Swiatek at Roland Garros in 2023 and reached the last four at Flushing Meadows for the second straight year with a 6-1, 6-4 win over No. 22 Beatriz Haddad Maia.

“It was me and me, it was my ghosts in my head and I know all tennis players have that,” Haddad Maia said. “Today was like an internal battle. I couldn’t do it.”

Muchova lost to champion Coco Gauff in the semifinals of the 2023 US Open, but had surgery on her right wrist in October and was off the tour for about 10 months, returning in June. It was the latest in a string of injuries for Muchova, who called it “one of the worst injuries I’ve had.”

“Looking back now,” she said, “I think, ‘Oh, the time really flew by, and I feel strong again.’”

The other women’s singles match on Thursday features Australian Open champion Aryna Sabalenka, seeded second, against debuting 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 quarter-finals, No. 25 Jack Draper reached his first Grand Slam semi-final – and became the first Briton to reach that stage at the US Open since Andy Murray lifted the trophy in 2012 – by beating No. 10 Alex de Minaur 6-3, 7-5, 6-2.

The other men’s semifinal will feature No. 12 Taylor Fritz and No. 20 Frances Tiafoe, the first all-American match at a major tournament in 19 years.

Draper has won all 15 sets he has played so far, but things look set to get even tougher on Friday, when he will face either No. 1 Jannik Sinner or No. 5 Daniil Medvedev for a place in the final. Medvedev won the title in New York in 2021 and was the only former champion in the men’s pool going into his showdown with Sinner in the final singles quarterfinal on Wednesday.

“This isn’t something I’m going to do overnight. I’ve believed for a long time that I’ve been working hard and doing the right things, and I knew my time was coming,” said Draper, whose right thigh 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 I can do a lot of great things from here. I’m very proud of myself.”

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