Venus Williams was beaten in the third round of the French Open by 15-year-old Bulgarian Sesil Karatantcheva.
Williams, runner-up at Roland Garros in 2002, slumped to a 6-3 1-6 6-1 defeat to Karatantcheva, who was the French Open junior champion last year.
Eleventh seed Williams lost the first set 6-3 but looked to be back on course when she raced through the second 6-1.
But Karatantcheva held her nerve while the American capitulated in the deciding set, not holding serve once.
Karatantcheva will now play the winner of the match between Frenchwoman Emilie Loit and Swiss player Emmanuelle Gagliardi.
There were six breaks of serve in the first set before Karatantcheva finally clinched it 6-3.
Williams had played erratically but raised her level to break three times in the second.
But she again fell apart in the third and the end came when she put another volley wide.
In all, Williams lost her serve nine times in the match against Karatantcheva, who is ranked 98 in the world and playing her first senior French Open.
Williams described her defeat as "very disappointing" but vowed to bounce back.
"Maybe I went for it too much - basically there were just too many errors," said the 24-year-old.
"I had never heard of her before or seen her but if I had played just 10% better, I would have won.
"I kind of beat myself. But I still feel like I can be a great player again."
Williams, who came into the tournament on the back of her first tournament win in a year at the Istanbul Cup, has won four Grand Slam events but none since 2001.