Wimbledon champion Maria Sharapova made an early exit at the US Open as she was outfought by a rejuvenated Mary Pierce.
Twice former Grand Slam winner Pierce advanced 4-6 6-2 6-3 after an enthralling encounter.
Sharapova has struggled for form since winning Wimbledon but she looked on course when she took the first set.
But Pierce fought her way back into the match with some brutal returning and earned the crucial at 4-3 in the decider before serving it out.
The Frenchwoman will play 10th seed Svetlana Kuznetsova for a place in the quarter-finals.
Sharapova, 17, insisted she was not too downcast after her defeat, adding that she had plenty to learn despite her Wimbledon win,
"She's an experienced player. I have to move on and there are so many things I have to work on," said the Russian.
"I tried to do too much at certain times - I went too much on my serve at the wrong time.
"It's not the end of the world, I only lost a tennis match and there are far more important things going on in the world today," added Sharapova, who wore a black ribbon in memory of the victims of the Beslan school siege.
Pierce, who has been beset by injuries in recent years, said she was elated to put those problems behind her.
"This feels really good after all I've been through," said the former Australian and French Open champion.
"There was a time with my back problem that I thought I wouldn't be able to play tennis again, so these moments are all the sweeter.
"I just love what I do so much more now than I did before."