As her second-round match against Garbiñe Muguruza on a sweat-soaked Thursday afternoon dragged on, Johanna Konta caught herself glancing at the courtside clock.

“Oh, O.K.,” Konta recalled thinking. “We’ve been here for a while.”

The longest women’s match in United States Open history consumed 3 hours 23 minutes on Court 17, where Konta, a 24-year-old Briton, nibbled on bananas. She recalled the teachings of Juan Coto, her mental coach. And she ultimately persevered through the conditions to oust the ninth-seeded Muguruza, 7-6 (4), 6-7 (4), 6-2.

“I left everything out there,” said Konta, ranked No. 97.

Fans who gathered at the U.S.T.A. Billie Jean King National Tennis Center might have expected the tournament to settle into a predictable rhythm after three days of relative chaos and scorching temperatures, but Konta was among the players who continued to surprise.

American women again made their mark: Seven were assured of advancing to the third round, including Shelby Rogers, a 22-year-old qualifier ranked 154th, who defeated Kurumi Nara of Japan in straight sets.

PhotoJohanna Konta outlasted Garbiñe Muguruza on Thursday in the longest women’s match in U.S. Open history.Credit Ben Solomon for The New York Times

Among the American men, No. 13 John Isner and Donald Young won second-round matches. Isner defeated Mikhail Youzhny in straight sets, while Young — a onetime prodigy who has found midcareer stability at age 26 — again played well, easing past Aljaz Bedene, 3-6, 6-4, 6-4, 6-2.

No. 28 Jack Sock was not as fortunate. Up by 6-4, 6-4, 3-6, 1-2 against Ruben Bemelmans, Sock appeared to faint early in the fourth set and was forced to retire as trainers tended to him.

Fitness has been a problem for Sock, who retired after three sets in his first-round match at last year’s Open because of cramps. On Thursday, his physical issues were dire.

After losing the third set to Bemelmans, Sock asked for a doctor and took some pills. He was able to play the next two games without obvious problems, but during his subsequent service game, he hunched over and leaned against his racket. He was assessed a point penalty for a time violation, which brought the match to its next changeover.

But Sock could barely move. As medical personnel piled ice bags on his body, Sock drooped to the court, and officials stopped the match.

PhotoAndy Murray rallied from two sets down to beat Adrian Mannarino in five sets.Credit Ben Solomon for The New York Times

“It’s not a pretty sight,” said Bemelmans, a Belgian ranked 107th. “I know how it feels when you’re cramping and dizzy.”

About three and a half hours after the match, Sock said in a statement that he was feeling better.

“Playing in the U.S. Open is the biggest and most important moment of the season for me, so having to retire from my match today is extremely disappointing,” he said.

Shortly after Sock’s incident on the Grandstand, Denis Istomin retired from his match against Dominic Thiem, bringing the total retirements in the men’s singles draw to 12, a record for a Grand Slam event in the Open era.

No. 3 Andy Murray survived a second-round scare, coming back from two sets down to defeat Adrian Mannarino of France, 5-7, 4-6, 6-1, 6-3, 6-1, at Arthur Ashe Stadium. Murray eventually found his form and needed just 22 minutes to polish off Mannarino in the fifth set.

PhotoVictoria Azarenka beat Yanina Wickmayer in two sets.Credit Ben Solomon for The New York Times

“I kept telling myself I would get there eventually and managed to turn it around,” he said, adding: “I’m proud of the way that I fought. Not an easy match to come through at all, and he was making it extremely difficult for me.”

Roger Federer made short work of his second-round opponent, Steve Darcis, winning by 6-1, 6-2, 6-1 in 1:20 and ripping 46 winners to just eight by Darcis, a Belgian veteran.

“I was pretty much all-out attack as much as I could,” Federer said.

Fourth-seeded Caroline Wozniacki also lost, falling in a marathon match, 6-4, 5-7, 7-6 (1), to 149th-ranked Petra Cetkovska in 3:02 in the last match of the night session.

Cetkovska, a crafty Czech who had upset Wozniacki at Wimbledon in 2013, led by a double break at 4-1 in the second set before her nerves and Wozniacki’s steely resolve extended the match into a third set.

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2015 U.S. Open

But Wozniacki, the Open runner-up last year, could not capitalize on the momentum, and she failed to convert any of her four match points. Cetkovska dominated the tiebreaker, which ended with Wozniacki hitting a backhand into the net.

“I said it’s now or never,” Cetkovska said in an on-court interview of saving the four match points. “And I just went for it.”

The upset of the afternoon belonged to Konta, who prolonged a string of recent disappointment for Muguruza, who has won only one match since reaching the final at Wimbledon in July.

“I think I am very tough on myself sometimes,” said Muguruza, who committed 59 unforced errors. “It’s hard when you lose. It’s always been like this.”

Muguruza said she could sense early in the match that Konta believed in herself, perhaps for good reason. In June, at a tuneup for Wimbledon, Konta defeated Muguruza in three sets. Konta later had the misfortune of facing Maria Sharapova in the first round at Wimbledon (Sharapova won easily), but Konta’s confidence was growing. She has not lost since.

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2015 U.S. Open Results: Day 3

Serena Williams takes the next step in her Grand Slam journey later today, facing Kiki Bertens. On the men’s side, top-seeded Novak Djokovic is the headliner on Arthur Ashe.

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