“For the first time in a while, my nerves really got the better of me, and it kind of became a little bit paralyzing,” Keys said after the match. “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.”

Madison Keys admitted nerves "paralyzed" her in New York as she suffered an unexpected US Open first-round defeat to unseeded Renata Zarazua.

American Madison Keys couldn't overcome unforced errors and double faults as Renata Zarazua made Mexican women's tennis history at the U.S. Open.

The sixth-seeded Australian Open champion is undone by 89 unforced errors, including 14 double faults, in a 6-7 (10), 7-6 (3), 7-5 loss to Mexico’s Zarazua, who needs to produce just eight winners

Pretty much from the get-go at the U.S. Open on Monday, Madison Keys could tell she wasn't hitting the ball well or feeling very much at all like the self-confident player who claimed her first Grand Slam title at the Australian Open in January.