Amelie Mauresmo's bid to regain the world number one 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 on Sunday.
In the men's tournament, top seed and defending champion Roger Federer began his campaign in the Masters Series event with a 6-3, 6-3 triumph over American Mardy Fish.
Linetskaya saved a match point at 4-5 down in the third set when the second seed from France pushed a forehand volley wide.
Playing in swirling winds that dusted the court with sand in this desert community, Mauresmo then double faulted to allow the Russian to break serve and lead 6-5.
Linetskaya, who lost to Mauresmo at the Australian Open in January, clinched victory when she put away a volley.
The 18-year-old screamed in delight after registering her first win over a top-five player.
Mauresmo had a chance to regain the number one ranking from Lindsay Davenport here but is now in danger of losing her number two spot to Maria Sharapova.
Mauresmo was not the only women's seed to fall.
Japan's Rika Fujiwara ousted number 24 Iveta Benesova of the Czech Republic 7-6, 7-6 and 17-year-old Russian Maria Kirilenko surprised number 28 Anabel Medina Garrigues of Spain 6-4, 6-4.
Fourth seed Elena Dementieva of Russia put out France's Severine Beltrame 5-7, 6-3, 7-6.
French Davis Cup hero Paul-Henri Mathieu upset 16th seed Tommy Haas of Germany 7-6, 6-3 in the men's draw.
Czech Tomas Berdych toppled number 19 Mario Ancic of Croatia 7-5, 3-6, 6-2 while another Czech, Jan Hernych, overcame number 20 Vincent Spadea of the U.S. 6-2, 1-6, 7-6.