Ukrainian wild card Elina Svitolina, who returned to the tour in April after giving birth to her daughter last October, ousted the top-seeded Swiatek on Centre Court 7-5, 6-7 (5), 6-2 on Tuesday to reach the semifinals at the All England Club for the second time in four years.
Pegula's hopes of finally reaching the semifinals of a Grand Slam event were burst after the fourth seed failed to win another game after leading 4-1 lead in the final set against Marketa Vondrousova, a left-handed Czech player who reached the French Open final in 2019.
Svitolina, 28, trailed 4-2 in the opening set and struggled to get her serve going in blustery conditions as Swiatek moved to a 5-3 lead. But after losing the next two points, momentum suddenly swung Svitolina's way as she went on to win 16 of the next 18 points to grab the first set and leave a stunned Swiatek wondering what had gone wrong during a 20-minute break to close the roof.
"Honestly, it's hard for me to, like, point (to) one thing what failed exactly," Swiatek said. "I think Elina was just overall playing aggressively and giving it all in every shot."
The interlude gave Swiatek a chance to reevaluate her tactics, and the four-time major champion fiercely fought to win the second-set tiebreak on an errant backhand by Svitolina.
Svitolina kept believing as she broke for a 2-1 lead in the final set and then was dominant the rest of the way to end Swiatek's 14-match winning streak.
"First of all I'm going to have a beer, probably," Svitolina said during her on-court interview. "At the beginning of the tournament, if someone would tell me that I will be in the semifinal and beating world No. 1, I would just say that they're crazy."
Svitolina says she now plays for more important reasons. For her daughter, Skaï, who was born in October. For her country, Ukraine, where a war that began with Russia's invasion in February 2022 continues to this day.
"War made me stronger and also made me mentally stronger," Svitolina said. "Mentally, I don't take difficult situations as, like, a disaster, you know? There are worse things in life. I'm just more calmer.
"Of course, I want to win. I have this motivation, like huge motivation, to come back to the top. But I think having a child -- and war -- made me a different person. I look at the things a bit differently."
It was her seventh career victory against a world No. 1 player, which ranks second among active players to Venus Williams (15).
Svitolina, who became just the third woman to defeat four former major champions in a Grand Slam event, will next face Vondrousova.
"I'm just going to enjoy tonight and then get some treatment, get some massage, much needed and then just regroup and be ready for the next big battle," said Svitolina, who is ranked No. 76 in the world and entered Wimbledon with 100-1 odds to win.
Vondrousova reached the semifinals by winning five straight games in the third set to close the match against Pegula.
"I'm just trying to stay in every game. It can change pretty quickly," Vondrousova said. "She got so much better in the second set. She was pushing me today."
Pegula is now 0-6 in Grand Slam quarterfinals. The 29-year-old American was playing at this stage at Wimbledon for the first time, and her Tuesday continued to go downhill as she and doubles partner Coco Gauff, the No. 2 seeds, lost in the third round to the unseeded duo of Laura Siegemund and Vera Zvonareva 6-3, 6-3.
Vondrousova had won only four matches on grass heading into the tournament at the All England Club, but she has won five in a row on the surface over the last eight days.
"My best result here was second round. It's amazing. I'm just loving grass now," Vondrousova said.
The victory for Vondrousova is her fourth over a seeded player at this year's Wimbledon tournament. She beat No. 12 Veronika Kudermetova in the second round, No. 20 Donna Vekic in the third round and No. 32 Marie Bouzkova in the fourth round.
Pegula was leading 3-1 in the third set when play was suspended so that the roof over No. 1 Court could be closed with rain approaching. Pegula then held serve in the next game to lead 4-1 before Vondrousova took over.
The Associated Press and Reuters contributed to this report.