NEW YORK -- Australian Open champion Madison Keys was undone by 89 unforced errors and was upset in the first round of the US Open 6-7 (10), 7-6 (3), 7-5 by Mexico's Renata Zarazúa on Monday.
The sixth-seeded American made so many mistakes, including 14 double faults, that Zarazúa needed to produce just eight winners to earn the biggest victory of her career.
Zarazúa, ranked No. 82, had lost in either the first or second round in all eight of her previous Grand Slam appearances. She became the first Mexican player to knock off a top-10 seed at a major since Angélica Gavaldón, who defeated No. 3 seed Jana Novotná in the round of 16 at the 1995 Australian Open.
Before Keys, the last American woman seeded in the top 10 to lose in the first round at Flushing Meadows was No. 8 Chanda Rubin in 2003, to María Vento-Kabchi. It's the only other instance of it happening at the New York tournament in the past 30 years.
"For the first time in a while ... my nerves really got the better of me, and it kind of became a little bit paralyzing," said Keys, the runner-up in New York to good friend Sloane Stephens in 2017 and a semifinalist in 2018 and 2023. "I felt like I was just slow. I wasn't seeing things the way that I wanted to, which I feel like resulted in a lot of bad decisions and lazy footwork."
Keys earned her first major championship at Melbourne Park in January, defeating No. 1 Aryna Sabalenka in the final.
"You always kind of feel first-round jitters and, as the day is getting closer, feeling a little bit more and more nervous," said Keys. "But I feel like, for whatever reason, today I just couldn't separate myself from ... feeling like winning matters just way too much."
Playing with her left thigh heavily taped, the big-hitting Keys struggled for stretches in Arthur Ashe Stadium against the 5-foot-3 Zarazúa, who came into the day with a 0-6 record against opponents ranked in the top 10.
"I'm a little bit small in height, so coming in here, it was like: 'Oh, my god. This is huge,'" Zarazúa said about the largest stadium in Grand Slam tennis, which holds nearly 24,000 spectators.
"When I retire, I'm going to be really happy about it," Zarazúa said, "so I was like, 'Just enjoy it.'"
This one certainly was memorable, in part because it did not come easily and lasted 3 hours, 10 minutes.
Zarazúa trailed by a set -- after frittering away five chances to take the opener -- and 3-0 in the second. But she never went away.
"Kudos to her for making me play a lot of balls today," Keys said. "I mean, she's a tricky player."
While Keys was one of 25 American players in the women's singles draw, the 27-year-old Zarazúa is Mexico's lone entrant in the bracket. She moved to San Antonio as a teen and is now based in Florida.
"In Mexico, yeah, it's probably not the most famous sport," Zarazúa said about tennis, which she picked up after starting out in gymnastics and diving.
"I got into tennis," she explained, "just because my brother was playing, and he was like, 'I think this is a bit safer sport.'"
In other women's singles results Monday, two-time major champion Barbora Krejcikova took down 18-year-old Canadian Victoria Mboko, an emerging star seeded 22nd who was making her US Open main draw debut fresh off her title run at Montreal, where she defeated four former major champions en route to her first WTA title.
Krejcikova is unseeded at Flushing Meadows, the first time she has been unseeded at a Slam since the 2021 French Open, where she went on to win her first major singles title.
Belgium's Elise Mertens, the No. 19 seed, beat American Alyssa Ahn 6-1, 6-0, and No. 17 seed Liudmila Samsonova got past Yuan Yue 2-6, 6-4, 6-4.
Also advancing was No. 9 seed Elena Rybakina, a 6-3, 6-0 winner over 16-year-old American Julieta Pareja, the youngest player in either field at this year's US Open. Fellow American Taylor Townsend moved on in straight sets against Antonia Ruzic.
Two-time Wimbledon champion Petra Kvitova, who is 35, played her last match before retirement, losing to Diane Parry 6-1, 6-0. Caroline Garcia, a 31-year-old from France, also exited the final tournament of her career, eliminated 6-4, 4-6, 6-3 by Kamilla Rakhimova.
"I'm very happy. It was a great run. I did a great thing on court, and I went until what I could achieve," said Garcia, a 2022 semifinalist in New York. "Obviously you can always achieve more, and I was dreaming of achieving more. But I'm very happy and [at] peace with my decision to move forward with my life and close the chapter of being a tennis player."
ESPN Research and The Associated Press contributed to this report.