Two-time Grand Slam champion Garbine Muguruza has been knocked out of the US Open, losing in straight sets to Tsvetana Pironkova.
The Spanish tenth seed, who reached this year's Australian Open final, lost 7-5 6-3 at Flushing Meadows.
Pironkova, 32, is playing her first tournament since Wimbledon 2017, after which she gave birth to her first child.
The Bulgarian will play Croat 18th seed Donna Vekic next in New York.
Elsewhere, Australian Open champion Sofia Kenin beat Canada's Leylah Fernandez 6-4 6-3.
The 21-year-old American, seeded second, will face Tunisian 27th seed Ons Jabeur in the third round.