Mexico's Renata Zarazua pulled off a stunning upset by beating sixth seed Madison Keys, while American Frances Tiafoe launched a bid to end the American men's drought at the US Open with a confident win over Japan's Yoshihito Nishioka.
Keys finally claimed her maiden major at the Australian Open this year but the American produced a staggering 89 unforced errors in the first match of the day on Arthur Ashe Stadium as she became the highest seed in the tournament to fall in a 6-7(10) 7-6(3) 7-5 defeat.
Zarazua, the only Mexican player in the US Open main draw, saved seven break points across the match for her first win over a top-10 player in a gruelling, three-hour and 10-minute affair on Monday.
"Coming into the match I was like almost crying because I was really nervous but I think the crowd made it so chill for me," she said. "I could hear some Mexican (fans) cheering so that was very nice."
Keys, 30, said the jitters cost her dearly in her first US Open appearance as a Grand Slam winner against her unseeded opponent.
"I feel like today ... for the first time in a while, my nerves really got the better of me, and it became a little bit paralysing," Keys told reporters.
"I felt like I was just slow, I wasn't seeing things the way that I wanted to which I feel resulted in a lot of bad decisions and lazy footwork.
Zarazua will hope to recreate the magic in the second round, where she faces Frenchwoman Diane Parry, who had no problems eliminating the Czech Petra Kvitova 6-1 6-0.
The twice semi-finalist joins fourth seed Taylor Fritz and sixth seed Ben Shelton in the second round, as they aim to become the first American man to lift the US Open trophy in 22 years. He plays US qualifier Martin Damm next.
Twice major winner Barbora Krejcikova knocked out Victoria Mboko6-3 6-2 in the first match on Louis Armstrong Stadium after missing several months injured this year, weeks after the promising Canadian teenager's career breakthrough in Montreal.
She next plays Japan's Moyuka Uchijima.
Last year's semi-finalist Jack Draper dismissed Argentine qualifier Federico Agustin Gomez 6-4 7-5 6-7(7) 6-2 in the second match on Armstrong and plays Belgian Zizou Bergs next.
The British fifth seed is joined in the second round by compatriot Cameron Norrie, who advanced after American Sebastian Korda retired when trailing 7-5 6-4. He will next play Argentinian Francisco Comesana.
The 19-year-old Brazilian fan favourite Joao Fonseca brought thrills to the Grandstand crowd as he kept his dream 2025 season on track with a 7-6(3) 7-6(5) 6-3 win over Serbia's Miomir Kecmanovic, setting up a meeting with 21st seed Tomas Machac.
Venus Williams showed flashes of vintage form under the lights at Arthur Ashe Stadium, but despite pushing Karolina Muchova to a deciding set the 45-year-old American fell 6-3 2-6 6-1 in a thrilling first-round battle.
The seven-times Grand Slam champion was making a record-extending 25th main draw appearance in New York, where she won the title in 2000 and 2001, and delivered a spirited performance for two hours before a packed and partisan crowd.
"I was stressed," Muchova said in her on-court interview. "You guys make me stressed, it was an unbelievable atmosphere. She's such a legend so it's nice to share the court with her. I'm so happy I was able to win today."
Williams, whose last US Open appearance ended in a 6-1 6-1 defeat by Greet Minnen in 2023, enjoyed thunderous applause with every point she won and left the court to a warm ovation from fans.
Kvitova says goodbye
The 35-year-old returned to competition earlier this season following a 17-month maternity break and announced ahead of Wimbledon that she would call time on her career at the end of the year's final major at Flushing Meadows.
Kvitova won 31 titles - including Wimbledon crowns in 2011 and 2014 - reaching a highest ranking of No 2 in the world, but will also be remembered for her brave return to tennis after a serious hand injury sustained during a home robbery in 2016.
After losing to Parry on Monday she was teary during an on-court ceremony to honour her.
"I think I would be proudest of many things, especially the mental side. All the seasons, pretty long seasons. I was okay to handle it even with some injuries and sickness," she told reporters later.
"I'm very proud of how I handled the pressure, how many times I have been in the top 10. It was special. I was never world number one but those two Grand Slam wins are above the world number one ranking.
"That's how I'm going to take it, especially the Wimbledon ones. I'm proud of many things."
