Australian Open champion Madison Keys says nerves got the better of her "for the first time in a while" after a surprise defeat by world number 82 Renata Zarazua in the US Open first round.

Sixth seed Keys, competing in New York for the first time since becoming a Grand Slam champion, put in an error-strewn performance in a 6-7 (10-12) 7-6 (7-3) 7-5 defeat by the Mexican.

The American made 89 unforced errors, hit 14 double faults and saved just four of the 10 break points she faced.

"I feel like today, for the first time in a while, my nerves really got the better of me, and it kind of became a little bit paralysing," Keys said.

"You always kind of feel first-round jitters, and as the day gets closer, you're feeling more and more nervous."

Keys started the year in superb fashion, beating two-time defending champion Aryna Sabalenka in Melbourne to win a first Grand Slam title.

But after reaching the quarter-finals of the French Open, the 30-year-old suffered a surprise third-round Wimbledon exit to 37-year-old Laura Siegemund.

In New York, Keys made 37 unforced errors in the first set alone but managed to take it on a tie-break after Zarazua spurned five set points.

The match looked there for the taking for Keys as she breezed to a 3-0 lead in the next set, but the 2017 runner-up in New York then lost four games in a row.

She managed to take the set to a tie-break but double faults and errors helped Zarazua force a decider.

Keys' coach and husband Bjorn Fratangelo offered advice and encouragement throughout - at one point saying: "It's just a sport, nothing's riding on the line" - but the American's play remained erratic.

"I want to win, but [I was] just feeling like winning matters way too much. I just couldn't quite separate myself from that," Keys added.

"Once you start playing badly, it just kind of all snowballs.

"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."

Zarazua secured her first victory against a top-10 player when Keys netted on match point, sparking wild celebrations among the Mexican's support team after a contest lasting three hours and 10 minutes.

Mboko out but Rybakina eases through

Victoria Mboko's fairytale 2025 will not get the perfect ending after she lost 6-3 6-2 to 2024 Wimbledon champion Barbora Krejcikova.

The 18-year-old defeated four Grand Slam champions on her way to winning the Canadian Open earlier this month, and is now ranked 23rd in the world having begun the year down in 333rd.

But her inconsistency cost her at Louis Armstrong Stadium as the teenager struggled on second serve and committed 10 double faults.

Krejcikova looked much like her old self after slipping to 62nd in the rankings during an injury-hit year.

Elsewhere, ninth seed Elena Rybakina handed a harsh lesson to 16-year-old Grand Slam debutant Julieta Pareja, wrapping up a 6-3 6-0 win in one hour and two minutes.

The world junior number one held her own in the opening stages, but soon succumbed to the 2022 Wimbledon champion looking to progress beyond the US Open third round for the first time.

American Pareja earned a wildcard spot after reaching the semi-finals in Bogota as a qualifier in her WTA debut, before finishing runner-up in both singles and doubles at this year's Wimbledon juniors event.