Romanian teenager Sorana Cirstea upset third seed Anna Chakvetadze 6-2, 1-6, 6-2 in the second round of the Family Circle Cup on Tuesday.
It was the first WTA Tour victory over a top 10 player for the 18-year-old, who won a three-set match against American Vania King on Monday and is ranked 86th in the world.
"Until now, I was saying hardcourt was my favourite surface but I think I'm going to change my mind," a beaming Cirstea told reporters.
"I like the green clay better than red clay. It really suits my game because I am really aggressive.
World number six Chakvetadze had a first-round bye.
"She just pushed the ball back and I made mistakes," the Russian said.
Sixteenth seeded Michaella Krajicek of the Netherlands also lost, going out 6-3, 3-6, 6-4 to Alize Cornet of France in a first-round match.
Slovakia's Dominika Cibulkova, the 15th seed, retired with a back injury trailing 6-4, 1-0 in a first-round encounter with Anastasia Rodionova of Russia.
In second-round action, fifth-seeded American Serena Williams brushed aside Argentina's Gisela Dulko 6-3, 6-4 and Frenchwoman Marion Bartoli, the sixth seed, scraped past Casey Dell'Acqua of Australia 7-5, 4-6, 6-1.
Eighth-seeded Dinara Safina of Russia swept past Spaniard Lourdes Dominguez Lino 6-1, 6-4 and 10th seed Agnes Szavay of Hungary edged Ai Sugiyama of Japan 7-5, 3-6, 6-4.
Szavay will next meet Safina who reached the Family Circle Cup final in 2007 where she lost to Serb Jelena Jankovic.
"(Dominguez Lino) had already played one match in the tournament and the key for me was to not give her any time," Safina said.
Top seed Jankovic and second-seeded Russian Maria Sharapova received first-round byes and will play on Wednesday. Jankovic faces American qualifier Alexandra Stevenson and Sharapova takes on American Bethanie Mattek.
