Ana Ivanovic advanced to her first Bank of the West quarterfinal, beating qualifier Carol Zhao 6-1, 6-1 on Thursday.
The 11th-ranked Serb, looking for her fourth title of the year, will face top-ranked Serena Williams on Friday.
Zhao, a sophomore All-American at Stanford, appeared in her second WTA Tour event and gained her first career win at the level Tuesday when Yanina Wickmeyer retired because of a viral infection.
Ivanovic, a former No. 1, had three aces and won 75 percent of her first-serve points.
In other matches, third-seeded Angelique Kerber beat Coco Vandeweghe, 7-6(4), 0-6, 6-2, and Garbine Muguruza topped Daniela Hantuchova, 6-4, 6-4.