Amelie Mauresmo's attempt to regain the world No 1 ranking took a knock when she lost 2-6, 6-2, 7-5 to Russian teenager Evgenia Linetskaya in the third round of the Pacific Life Open.
Linetskaya saved a match point at 4-5 down in the third set when the second seed from France put a forehand volley wide. Mauresmo double-faulted to allow the Russian to break serve and lead 6-5. Linetskaya clinched victory with a volley.
The 2003 champion Kim Clijsters looked impressive in her comeback from wrist surgery, defeating the Russian Anna Chakvetadze 6-1, 7-6.
In the men's tournament, the top seed and defending champion Roger Federer began his campaign in the Masters Series event with a 6-3, 6-3 second round triumph over the American Mardy Fish.
Federer has lost only once this year, to Marat Safin in the Australian Open semi-finals. Since then he has won titles in Rotterdam, Doha and Dubai.
"We'll really see after these two events, Indian Wells and Miami, how I'm really shaping up, if I'm still No 1," Federer said. "But so far I have the feeling that the chances are there to repeat last year's season."
The second seed Lleyton Hewitt defeated American Robby Ginepri 6-2, 7-6 while Safin survived two match points before overcoming Finland's Jarkko Neimenen 4-6, 6-1, 7-5.