Defending champion Carlos Alcaraz returned to singles action with a victory after a 139-day absence, while Aryna Sabalenka began her bid for a third consecutive U.S. Open title with a winning start Monday, providing plenty of star power on the second day of the year's final Grand Slam.
Former champion Iga Swiatek and last year's runner-up Amanda Anisimova also advanced with convincing victories, while Stefanos Tsitsipas delivered one of the day's biggest upsets by knocking out No. 10 seed Arthur Fils on a sunny afternoon in New York.
Alcaraz entered the tournament with questions surrounding his fitness after spending nearly four and a half months sidelined by a wrist injury. But the Spaniard quickly eased those concerns, cruising past Russia's Roman Safiullin 6-4, 6-4, 6-4 in his first competitive singles match since his injury.
"You don't know what's going to happen in the match after 4 1/2 months almost," Alcaraz told reporters. "You don't know what's going to happen in your body, in your mind, everything. It's totally different.
"But I just tried to play one game at a time and tried to feel good, to enjoy playing points, feeling the love from the people. Hopefully they enjoyed it."
The second seed's victory ensured some star power remained in the men's draw.
Novak Djokovic's shock defeat Sunday left another hole in a men's draw already without injured world No. 1 Jannik Sinner.
Earlier at Arthur Ashe Stadium, Sabalenka looked on course to end a frustrating patch of poor form with a 6-2, 6-4 win over Colombia's Camila Osorio that earned her a second-round clash with Russia's Polina Iatcenko.
"I'm happy with my performance," said Sabalenka, who has not reached a title match since triumphing at Indian Wells and Miami to complete the Sunshine Double in March.
"She's a tough opponent, makes you work really hard to get every point."
American Anisimova, whom Sabalenka beat in last year's final, got her campaign off to a good start on her 25th birthday with a 6-1, 6-3 win over compatriot Ashlyn Krueger at Louis Armstrong Stadium.
Greece's Tsitsipas upset Cincinnati champion Fils 4-6, 7-6(3), 6-1, 6-4 in an early match on the same court, booking a second-round meeting with South African Lloyd Harris.
"It's very easy to not feel confident, especially when you haven't beaten top players in a long time," Tsitsipas said. "I'm ecstatic about today. It's an important, good victory for me, for my confidence."
Seven matches that were suspended on the outer courts Sunday because of rain resumed Monday, as organizers benefited from the extended three-day first-round schedule after the tournament expanded to 15 days in 2025.
Wimbledon finalist Karolina Muchova got the day underway on Grandstand with a routine 6-3, 6-0 win over Sinja Kraus, and the Czech seventh seed will play Britain's Katie Boulter next.
Wimbledon champion Linda Noskova, who defeated compatriot Muchova in the final, beat American Katie Volynets 7-5, 6-1 to stay on track for back-to-back major titles.
It was a good day for the Americans on the largest of the outdoor courts. Emma Navarro ground out a 7-6(6), 6-7(1), 6-3 win over Frenchwoman Lois Boisson, while Taylor Townsend defeated Poland's Maja Chwalinska 6-2, 6-3.
China's Olympic champion Zheng Qinwen, who was forced to go through qualifying only two years after reaching the quarterfinals in New York, beat Kristina Liutova 4-6, 6-3, 6-1.
Zheng's progress has stalled and her ranking has plummeted since an elbow injury forced her to undergo surgery last year.
Polish eighth seed Swiatek, the 2022 champion, beat China's Wang Xiyu 6-2, 6-3 to kick off the night session on the second showcourt.
Another former champion, Stan Wawrinka, fell to Italy's Matteo Berrettini 7-6(4), 7-6(3), 6-0. The 41-year-old Swiss bid farewell to the Grand Slam for the final time before his retirement at the end of the season.
Home hope Ben Shelton overcame Dutchman Tallon Griekspoor 1-6, 6-1, 7-6(3), 6-2 before Frances Tiafoe prevailed 6-4, 4-6, 3-6, 6-3, 6-4 in an all-American clash with Martin Damm.
Naomi Osaka unveiled another bold Grand Slam walk-on outfit before showing plenty of fight for a 7-6(6), 7-6(3) victory over unseeded Russian Anastasia Zakharova in a late-finishing match.
