Coco Gauff reacts during her win over Mirra Andreeva during the second round of the U.S. Open on Wednesday in New York. John Minchillo/Associated Press

NEW YORK — Novak Djokovic and Iga Swiatek rolled to straight-set victories to move into the third round at the U.S. Open on Wednesday, fighting not only their opponents but sticky, 85-degree 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.

“I still have the hunger,” said the 36-year-old from Serbia who owns 23 Grand Slam titles. “I still got it.”

Swiatek, the top-seeded defending women’s 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.

Ending the four-hour match with a forehand winner that just caught the baseline, Stricker raised his hands and flopped on his back to celebrate his first victory over a top-10 player, in only his third Grand Slam appearance.

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“I’m just super happy right now,” Stricker told the Grandstand crowd. “I came out and felt good from the first set on. … I just kept playing high-level tennis.”

Tsitsipas, a Grand Slam finalist at the Australian Open and French Open, has never advanced beyond the third round at Flushing Meadows. The Greek is the second-highest seed to go down on the men’s side after No. 4 Holger Rune of Denmark lost to 63rd-ranked Spaniard Roberto Carballes Baena.

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.

“I learned to be aggressive,” the 19-year-old Gauff said in her on-court interview. “I did well making her play off her back foot.”

Gauff next faces 32nd-seeded Elise Mertens of Belgium, who came back to beat American Danielle Collins, 3-6, 7-6 (9), 6-1. Gauff has won 13 of her last 14 matches, including claiming the two biggest titles of her still-nascent career.

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“Obviously, maturity plays a part into it. The more matches you have, the more experience you have,” said Gauff.

Two American men, No. 10 Francis Tiafoe and No. 14 Tommy Paul also advanced, but in quite different style. Tiafoe routed Sebastian Ofner,  6-3, 6-1, 6-4, while Paul rallied from two sets down to beat Roman Safiullin, 3-6, 2-6, 6-2, 6-4, 6-3.

Dominic Thiem, the 2020 U.S. 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 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.