American Varvara Lepchenko surprised top-seeded Caroline Wozniacki 6-4, 6-2 on Thursday night in the second round of the Bank of the West Classic.
The fifth-ranked Wozniacki, the Danish player who reached the round of 16 at Wimbledon, became the fourth seeded player to drop out and the second No. 1 seed. Top-ranked Serena Williams pulled out with an elbow injury on the eve of the draw.
Lepchenko will face Germany's Mona Barthel on Friday in the quarterfinals. Barthel advanced Wednesday.
Earlier, second-seeded Agnieszka Radwanska of Poland beat Japanese qualifier Misaki Doi 1-6, 6-2, 6-0 to set up a match against fifth-seeded Angelique Kerber of Germany. Kerber, a finalist at Stanford last year, beat Ana Konjuh of Croatia 6-3, 6-4.
Fourth-seeded Karolina Pliskova of the Czech Republic topped Japanese qualifier Kimiko Date-Krumm, at 44 the oldest player on the tour, 7-5, 6-2. Pliskova will face Croatia's Ajla Tomljanovic.
In the other quarterfinal, American Alison Riske will play eighth-seeded Elina Svitolina of Ukraine.