Nadia Petrova upset US Open champion Sam Stosur in the third round of the BNP Paribas Open on Monday.
The Russian, seeded 30, pulled off a shock 6-1 6-7 (6/8) 7-6 (7/5) win the sixth-seeded Australian, who has struggled this season following her triumph at Flushing Meadows last September.
Petrova served 15 aces during the match before clinching victory after two hours and 46 minutes on court.
World number four Caroline Wozniacki overcame a scare as she battled back from a set down to beat Sweden's Sofia Arvidsson 3-6 7-5 6-2 in two hours and 37 minutes.
The Dane was outplayed in the first set and teetered in the second before her class finally told in the third.
Second seed Maria Sharapova showed glimpses of her best form as she defeated Simoa Halep 6-3 6-4.
She admitted she was trying a little too hard towards the middle of the second set.
"I almost started going for a little bit too much," she said. "Instead of being patient, putting a few more balls back, I just hit a few errors that I shouldn't have made.
"I got it together in the end."
Tenth seed Francesca Schiavone was forced to withdraw after one set of her match with Lucie Safarova, which she had lost 6-2. The Italian is the latest victim of the stomach virus which has swept through the event and accounted for a number of players.
Seventh seed Marion Bartoli needed only 71 minutes to dispatch South Africa's Chanelle Scheepers 6-2 6-0, while there were also wins for Maria Kirilenko, Roberta Vinci and Ana Ivanovic.