China's Shaui Peng upset seventh seed Kim Clijsters 6-4 6-4 to reach the semi-finals at the WTA Acura Classic.
Peng, the world number 46, shocked Clijsters, who had won 26 hardcourt matches in a row in the US.
But the 19-year-old Peng, who upset third seed Elena Dementieva of Russia in the second round, continued her unlikely run.
Mary Pierce made smooth progress into the last four with victory over fifth seed Patty Schnyder.
Pierce, seeded sixth, saw off Schnyder 6-4 6-3 and now meets in the semi-finals.
Japan's Ai Sugiyama followed up Thursday's shock win over second seed Svetlana Kuznetsova by beating Sesil Karatantcheva 6-1 6-2.
She will now meet fellow countrywoman Akiko Moriogami, who defeated Anna Chakvetadze 6-3 6-4.