Casper Ruud, the 2022 US Open runner-up, was eliminated when Zhang Zhizhen became the first Chinese man to beat a player in the top five of the ATP rankings.
Zhang upset the No. 5 seed 6-4, 5-7, 6-2, 0-6, 6-2 to reach the third round of a Grand Slam tournament for the second time. It was his second five-set victory to open this tournament after he lost his first four five-setters.
He wasn't discouraged by those previous losses, with the breakthrough finally coming on Monday when he outlasted J.J. Wolf in 4 1/2 hours.
Zhang became the first Chinese man to compete in the main draw at Wimbledon in the professional era when he qualified in 2021. He qualified last year at the US Open, joining Wu Yibing as the first men from their country in the main draw of the year's final Grand Slam tournament.
The 26-year-old Zhang reached the third round of the French Open in June before falling to Ruud, who had been to the finals in three of the last six Grand Slam events.
Ruud, who lost to Carlos Alcaraz last year in Flushing Meadows, is the highest-ranked Norwegian in the history of the ATP rankings, which date to 1973. He had reached No. 2 last year and is currently No. 5.
But after dropping the fourth set in just 23 minutes, Zhang broke Ruud's serve to open the final set, adding a second for a 4-1 lead.
Zhang will face Rinky Hijikata of Australia in the third round.
Meanwhile, Caroline Wozniacki clearly made the right choice to come out of retirement and try her hand at tennis again after 3 1/2 years, making it to the third round at Flushing Meadows by beating two-time Wimbledon champion Petra Kvitova 7-5, 7-6 (5) in Arthur Ashe Stadium.
The 33-year-old from Denmark won the 2018 Australian Open and reached No. 1 in the rankings - she's currently No. 623 - during her first stint on tour. Wozniacki left the sport in early 2020 to start a family, and later spent time working as a TV analyst, and now she and former NBA player David Lee are the parents of two children.
She returned to competition this month and now is back at Flushing Meadows for the first time since 2019.
Wozniacki was the runner-up at the US Open in 2009, when she lost to Kim Clijsters in the title match, and 2014, when she lost to her good friend Serena Williams.
Next up will be a match against 2020 US Open semifinalist and 2021 Australian Open finalist Jennifer Brady, an American who recently came back to the tour after two years off because of injuries.
Novak Djokovic and Iga Swiatek rolled to straight-set victories to move into the third round, fighting not only their opponents but sticky, 85-degree Fahrenheit (29-degree Celsius) weather on the hottest day at the tournament so far.
"That's why we train several hours a day in humid, hot conditions so we can be ready for whatever awaits you on court," the second-seeded Djokovic said after a 6-4, 6-1, 6-1 victory in Arthur Ashe Stadium over 76th-ranked Bernabe Zapata Miralles of Spain.
Swiatek, the top-seeded defending champion from Poland, had only slightly more trouble in her 6-3, 6-4 victory over 322nd-ranked Australian Daria Saville in Louis Armstrong Stadium. Swiatek has never lost in a major to a player outside the top 100.
In the biggest upset of the day, seventh-seeded Stefanos Tsitsipas fell in the second round to 128th-ranked Swiss qualifier Dominic Stricker 7-5, 6-7 (2), 6-7 (5) 7-6 (6), 6-3.
Earlier, Coco Gauff cruised into the third round with a 6-3, 6-2 victory over 16-year-old Russian Mirra Andreeva.
The sixth-seeded American approached the net 18 times, winning 15 of those points, and she credited that newfound net game after struggling to beat German Laura Siegemund in three sets in the first round.
Gauff next faces 32nd-seeded Elise Mertens of Belgium, who came back to beat American Danielle Collins 3-6, 7-6 (9), 6-1.
Elsewhere, Dominic Thiem, the 2020 US Open champion, retired soon after dropping a first-set tiebreaker to American Ben Shelton, and French wild card Benjamin Bonzi defeated 28th-seeded Christopher Eubanks 7-6 (6), 2-6, 6-2, 7-6 (7), ending the hopes of an American who made a surprising run to the Wimbledon quarterfinals.
Fourth-seeded Elena Rybakina advanced to the third round in a walkover when Australian Ajla Tomljanovic withdrew before the match; 10th-seeded Karolina Muchova beat Magdalena Frech of Poland 6-3, 6-3; and 15th-seeded Belinda Bencic of Switzerland downed British qualifier Yuriko Lily Miyazaki 6-3, 6-3.