British number one Johanna Konta suffered a shock second-round defeat to Fanny Stollar in the Charleston Open in South Carolina.
Sixth seed Konta, 26, lost 3-6 4-6 to the 19-year-old Hungarian qualifier and world number 219.
The world number 22, who had a bye in the first round, beat Stollar in the Fed Cup in February.
She lost 62% of points on her second serve and failed to force a break point in the second set.
"She is really tough to play against so I was coming here expecting nothing," said Stollar, who hit 28 winners and now plays Dutch number 12 seed Kiki Bertens.
"I was just going to play my game and focus on every point. I really just tried to keep my serve, break hers, and keep her behind the baseline."
Second seed Petra Kvitova also fell to a surprise defeat in the first clay court event of the WTA season, losing to fellow Czech Kristyna Pliskova 6-1, 1-6, 3-6.
The two-time Grand Slam champion eased through the opening set but, despite breaking Pliskova five times, she also made 44 unforced errors in a haphazard performance.
After her slow start, the aggressive Pliskova seized command to claim the second and the world number 77 broke twice in the decider to seal victory.
"My serve was horrible in the first set, so I had to focus on my serve and the rest of the game was better then as well," said Pliskova, who spurned six match points.
Pliskova will next play Russian Elena Vesnina, who beat American Taylor Townsend 6-4 6-1.
Meanwhile, German fifth seed Julia Gorges beat Kristie Ahn of the United States 2-6 6-4 7-6 to set up a match with Japan's Naomi Osaka, who won her first WTA title with victory at Indian Wells in March.
And last year's US Open finalist Madison Keys defeated Lara Arruabarrena of Spain 6-1 3-6 6-3 and will face Italian Camila Giorgi in the third round.