Former champion Angelique Kerber fell to a first-round exit against unseeded Kristina Mladenovic at the US Open as her dismal Grand Slam record this season continued.
The German, seeded 14th, arrived at Flushing Meadows without a victory in her two warm-up events and was beaten 7-5 0-6 6-4 by the world No 54 from France.
Kerber has now won just one Grand Slam singles match - at Wimbledon last month - since she reached the fourth round at the Australian Open in January.
Mladenovic, who has been ranked as high as world No 10 in 2017, saved 10 of 12 breaks points in the first and third sets combined as she reached the second round after two hours and 23 minutes of action.
The 26-year-old will next face countrywoman Fiona Ferro, ranked world No 74.
Meanwhile, second seed Ashleigh Barty avoided an early exit as the Australian came back to defeat Zarina Diyas 1-6 6-3 6-2 in New York.
Barty, who was overtaken by Naomi Osaka as world No 1 earlier this month, made 36 unforced errors in the first set against the world No 80 from Kazakhstan.
The French Open champion broke Diyas for the first time in the eighth game of the second set as she won three games on the bounce from 3-3.
Barty then broke twice in the deciding set to seal a second-round meeting against American Lauren Davis.
"I was happy with the way I was able to fight through after a pretty awful start," Barty said.
Third seed Karolina Pliskova overcame a tough examination against compatriot Tereza Martincova to win 7-6 (8-6) 7-6 (7-3) at the end of a match which featured 12 breaks of serve.
Pliskova, finalist in 2016, saved two set points in the first set tiebreak before she held her nerve to progress to a meeting against Mariam Bolkvadze of Georgia.