NEW YORK — Jessica Pegula pulled off a major upset of Iga Swiatek at the US 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.

Her win guarantees the US Open will feature multiple American men and women playing in the semifinal round, the first time that has happened since 2003 (Andre Agassi and Andy Roddick; Lindsay Davenport and Jennifer Capriati).

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.

Both Muchova and Pegula have yet to drop a set, the first time that has been done by multiple women at this stage of the tournament since 2014 (there were three that year: Serena Williams, Peng Shuai and Ekaterina Makarova).

Pegula repeatedly did what seemed nearly impossible to do lately against Swiatek, who counts the 2022 US Open among her five Grand Slam titles and has led the WTA rankings for most of the past 2½ 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.

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.

Pegula entered the day as the only player inside the WTA’s top 10 who hadn’t reached a major semifinal; she was 0-6 in her career in Slam quarterfinals, one loss shy of the worst major quarterfinal record by any woman in the Open era, shared by three players (0-7).

During that 0-6 rut, 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.

Before Pegula, the last American woman to defeat the world No. 1 at any major was Shelby Rogers, who knocked out Barty in the third round of the 2021 US Open.

Entering Wednesday, Swiatek held a 6-3 edge in her head-to-head matchups with Pegula. Their last meeting was at the WTA Finals in Cancún, where Swiatek defeated Pegula 6-1, 6-0 in the most lopsided championship match in the history of the year-end event.

ESPN Stats & Information and The Associated Press contributed to this report.