Olga Govortsova of Belarus upset second-seeded Samantha Stosur, 6-2, 6-4 Tuesday in the second round of the Bank of the West Classic.
Stosur was awarded a first-round bye, while Govortsova defeated German Julia Goerges during Monday's first round. Govortsova came into the tournament ranked 83rd and for the first time won back-to-back matches this year.
American Varvara Lepchenko and Urszula Radwanksa of Poland were both first- round winners. The sixth-seeded Lepchenko cruised past Portugal's Michelle Larcher De Brito 6-2, 6-4 in 1 hour, 19 minutes.
Radwanksa, the seventh seed, advanced to the second round in similar fashion, disposing of American Christina McHale, 6-1, 6-3.
American qualifier Coco Vandeweghe breezed past Monice Niculescu 6-0, 6-3, while Russian qualifier Vera Dushevina needed just over an hour to down Marine Erakovic of New Zealand by a 6-2, 6-1 count.
Tamira Paszek of Austria also advanced Tuesday after her opponent, Russian qualifier Alla Kudryavtseva, retired in the third set with Paszek holding a 6-1, 6-7 (4-7), 3-0 advantage. Daniela Hantuchova defeated Yanina Wickmayer, 6-2, 4-6, 6-0.
The winner of this tournament will receive $125,000.