Garbine Muguruza was a high-profile casualty at the Indian Wells Masters, where the third seed was miraculously beaten by Sachia Vickery.
Muguruza appeared destined to cruise into the third round on Friday – leading the American qualifier 6-2 3-0 and up 40-love in the fourth game.
However, the Spanish star and two-time grand slam champion was on the receiving end of an amazing comeback as Vickery emerged from the jaws of defeat at the WTA Tournament.
World number one and top seed Simona Halep had no such troubles en route to the next round.
Petra Kvitova extended her winning streak, while fifth seed Karolina Pliskova survived to fight another day.
MUGURUZA IN STUNNING COLLAPSE
Somehow, Muguruza farewelled Indian Wells following a 2-6 7-5 6-1 defeat against Vickery.
Muguruza had no cause for concerns – she was on the verge of taking a 4-0 lead in the second set.
But then it all fell apart. Vickery won seven of the next nine games as the 22-year-old defied the odds in the second round, eventually easing to the unlikeliest of wins after two hours, 12 minutes.
It was her first top-10 triumph as Vickery looks ahead to Naomi Osaka, who backed up her victory against former world number one Maria Sharapova by ousting Agnieszka Radwanska 6-3 6-2.
HALEP OVERCOMES SLUGGISH START
The Australian Open runner-up recovered from a shaky start to beat Kristyna Pliskova 6-4 6-4.
Playing just her third tournament of the year, Halep showed signs of rustiness but she was still too good for her Czech opponent.
Halep – who will next face Caroline Dolehide after the wildcard upset 30th seed Dominika Cibulkova – hit 18 winners and only 12 unforced errors, while she converted all six of her break point chances.
"I think I played good tennis, even if the previous days I didn't feel the ball that well," Halep told reporters. "But today was great that I could actually play a few rallies. I got the rhythm back, and I feel good now. I feel more confident."
PLISKOVA ESCAPES WITH WIN
After sister Kristyna lost to Halep, Karolina ensured one Pliskova remained in the tournament after digging deep to outlast Irina-Camelia Begu 7-6 (7-4) 6-1.
Pliskova, who will face 32nd seed Zhang Shuai in the next round, trailed 5-1 in the opening set but she saved a pair of set points before seeing off Begu.
KVITOVA CELEBRATES 14TH SUCCESSIVE VICTORY
The ninth seed matched the longest winning streak of her career after prevailing 6-7 (4-7) 7-6 (7-3) 6-4 over Yulia Putintseva.
It was a gruelling encounter lasting more than three hours as Kvitova extended her unbeaten run to 14 matches.
Two-time Wimbledon champion Kvitova endured a mixed day, hitting 66 winners but tallying 78 unforced errors and 18 double faults.
Awaiting Kvitova is 16-year-old wildcard Amanda Anisimova, who stunned 23rd seed Anastasia Pavlyuchenkova 6-4 6-1.