Second seed Amelie Mauresmo produced a shockingly error-strewn display as she lost to Evgenia Linetskaya in round three at Indian Wells.
The French player produced 45 unforced errors and nine double faults on her way to a 2-6 6-2 7-5 defeat.
The Russian teenager was hardly on top form herself, producing no first serve aces and clocking slow second serves.
But having broken Mauresmo in the 11th game of the final set, she only wasted one match point before going on to win.
Kim Clijsters kept her comeback from a wrist injury on track with a 6-1 7-6 (7-4) victory over Anna Chakvetadze, while Elena Dementieva and Svetlana Kuznetsova also progressed.
Mauresmo could have regained her world number one spot from Lindsay Davenport at this tournament, but is now in danger of losing her number two ranking to Maria Sharapova.
"It's a big disappointment, especially having played the way I did today," said Mauresmo.
"You can lose some matches, but as long as I felt that I played some good tennis and that the opponent won it, then it's okay.
"I don't really feel it was the case today."
Mauresmo was the biggest name casualty of the day, but by no means the only seed to fall.
Another Russian teenager, Maria Kirilenko, beat 28th seed Anabel Medina Garrigues 6-4 6-4, and 24th seed Iveta Benesova lost 7-6 7-6 to Japan's Rika Fijiwara.