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 Zaraza on Monday.
The sixth-seeded Keys, who was the runner-up at Flushing Meadows in 2017, made so many mistakes, including 14 double-faults, that Zaraza needed to produce just eight winners to earn the biggest victory of her career.
The 82nd-ranked Zaraza 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 Anglica Gavaldn, who defeated No. 3 seed Jana Novotna 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 Maria Vento-Kabchi. It's the only other instance of it happening at the New York tournament in the past 30 years.
Keys earned her first major championship at Melbourne Park in January, defeating No. 1 Aryna Sabalenka in the final. In addition to her run to the final in New York eight years ago before losing to good friend Sloane Stephens, Keys also was a semifinalist at the US Open in 2018 and 2023.
But with her left thigh heavily taped Monday, the big-hitting Keys struggled for stretches in Arthur Ashe Stadium against the 5-foot-3 Zaraza, 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," Zaraza 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," Zaraza said, "so I was like, 'Just enjoy it.'"
In other women's singles results Monday, two-time major champion Barbora Krejcikova took down 18-year-old Canadian Victoria Mboko, an emerging star 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.
Also Monday, 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.
Two-time Wimbledon champion Petra Kvitova was beaten 6-1, 6-0 by France's Diane Parry in what was the last tournament for Kvitova before retirement.
ESPN Research and The Associated Press contributed to this report.
