Japan's Ai Sugiyama recovered from a set down to shock second seed Svetlana Kuznetsova 2-6 6-4 6-3 at the Acura Classic in San Diego.
Sugiyama, who faces Sesil Karatantcheva in the quarters, kept the pressure on Kuznetsova and the 2004 US Open winner came up with a string of mistakes.
The result was one of several shocks, with Anna Chakvetadze beating ninth seed Elena Likhovtseva 6-3 6-1.
Meanwhile, Shuai Peng saw off 16th seed Dinara Safina 7-6 6-0.
Indian qualifier Sania Mirza's memorable tournament came to an end as she lost 6-2 4-6 6-4 to Japan's Akiko Morigami.
Belgium's Kim Clijsters led a fightback from the favourites in the later matches with her comfortable 6-1 6-3 win over Jelena Jankovic.
Fifth seed Patty Schnyder saw off Italy's Francesca Schiavone, while Mary Pierce beat compatriot Nathalie Dechy.