Top seed Venus Williams was stunned in the first round of the Memphis International by Czech qualifier Petra Kvitova on Tuesday, the 17-year-old left-hander rallying to defeat the defending champion 2-6, 6-4, 6-3.
The American world number eight eased through the opening set before her opponent, ranked 135 places below Williams, rallied to take the final two sets for the biggest victory of her young career.
"She mixed it up really well and I didn't return well," Williams told reporters after seeing her opponent convert three of four break point opportunities in the deciding set.
"She has a good serve and she played well. I'm disappointed I'm not going to be here the rest of the week."
Kvitova admitted she was nervous in the opening set of her first match against a top 10 opponent, but settled down in the second.
"I started to believe I could win in the second," she said. "In the first set, I didn't."
In another upset, second seeded Frenchwoman Tatiana Golovin was beaten 6-4, 6-4 by American world number 148 Bethanie Mattek.
"This year I've actually been playing pretty good," Mattek said.
"I was sick in Australia and I played an awesome match in Chile [in a loss to Jelena Kostenic]. I had match points there but it just didn't go my way."
The American raced to a 5-1 lead in the first set and was never threatened.
Third seed Shahar Peer was able to advance with a 6-2, 6-1 victory over American Ahsha Rolle, joining sixth seeded Caroline Wozniacki in the second round after the Dane dumped Venezuela's Milagros Sequera 6-0, 6-2.
In second-round matches, Olga Govortsov of Belarus rallied to defeat Renata Voracova of the Czech Republic 4-6, 6-4, 7-6 and seventh seeded Sophia Arvidsson of Sweden battled to a 1-6, 7-5, 6-3 win over Canada's Stephanie Dubois.
In the men's draw, fourth-seeded Radek Stepanek, number five Sam Querrey and number six Thomas Johansson all advanced.