American Meghann Shaughnessy saved a match point before upsetting fifth seed Vera Zvonareva in a third-set tie-break at the Bank of the West Classic.
Zvonareva looked on course to edge a tight third set but Shaughnessy produced an ace at the right time and went on to win 6-3 1-6 7-6 (8-6).
"I played three pretty good matches in qualifying and that helped," she said.
Shaughnessy now meets Daniela Hantuchova, who swept past Alexandra Stevenson 6-2 6-1.
Zvonareva said: "We both played well, but maybe she was a little bit lucky on the break points.
"You play every week, you win, you lose, you go to the next tournament. I just want to be back in the top 10."
Earlier, seventh seed Jelena Jankovic beat Marissa Irvin 6-2 6-4, and eighth seed Francesca Schiavone took care of Alina Jidkova 6-4 6-1.
India's Sania Mirza won a tough encounter against Eleni Daniilidou 7-6 (7-4) 2-6 6-3 and goes on to face Wimbledon champion Venus Williams.