Second seed Vera Zvonareva was beaten in the second round of the W&S Open in Cincinnati on Thursday.
The Russian, who was the runner-up at the event last year, succumbed 6-3 6-2 to Akiko Morigami of Japan.
World number 72 Morigami will now face Sania Mirza in the last eight after the Indian star battled past American Kristina Brandi 7-6 (7-5) 7-5.
Third seed Jelena Jankovic reached the quarter-finals with a 6-3 7-5 victory over Austrian Sybille Bammer.
Bethanie Mattek of the USA went through when France's Camille Pin retired with a sore throat when trailing 6-3 2-0.
Top seed Patty Schnyder and number four seed Daniela Hantuchova are already through to the quarters.