Argentine veteran Paola Suarez toppled ninth seed Dinara Safina 3-6, 6-3, 6-4 to move into the second round of the San Diego Classic on Monday.
Safina was not the only to seed to fall, rising Serbian Jelena Jankovic knocking out number 16 Maria Kirilenko of Russia 6-1, 6-4.
Fourteenth seed Ana Ivanovic of Serbia advanced, however, crushing Japan's Akiko Morigami 6-2, 6-1.
Australia's Samantha Stosur earned a meeting with top seed Kim Clijsters when she beat French veteran Nathalie Dechy 7-5, 6-4.
"I think I played a solid game, served well and played well under pressure," Stosur told reporters. "Nathalie played all right. It wasn't her best match, but in tennis, it's hard to do that every time. The way I played didn't allow her to play her game."
Former top-15 player Suarez has had a hard time recovering from a hip injury sustained last year, but the 30-year-old troubled Safina, who had difficulty keeping the ball in the court.
Suarez will next face qualifier Katerina Bondarenko of Ukraine, who ruined the WTA debut of 14-year-old American wild card CoCo Vandeweghe, the niece of former basketball player Kiki Vandeweghe, 6-4, 6-2.
Russian qualifier Vasilisa Bardina, 18, continued her fine form in taking down Meng Yuan of China 6-4, 6-2, and she will next face second seed and compatriot Maria Sharapova.
Spanish qualifier Virginia Ruano Pascual defeated Elena Likhovtseva 6-4, 6-3 and will now play 17-year-old Czech Nicole Vaidisova, the seventh seed.
Austria's Sybile Bammer beat Czech Lucie Safarova 6-4, 6-7, 6-1 and will take on defending champion Mary Pierce, who has not played since February due to a foot injury.
Qualifier Meilen Tu overcame American Laura Granville 6-4, 1-6, 6-2 to set up a clash with former champion Martina Hingis.