Russia's Nadia Petrova sent defending champion Justine Henin-Hardenne crashing out of the US Open with a solid 6-3 6-2 victory in New York.
Both players traded four straight breaks in a scrappy start but it was Petrova who settled first and went on to break the world number one at 5-3.
Henin-Hardenne showed glimpses of her past dominance but looked anxious as she committed unforced errors.
Petrova broke the Belgian twice in the second and held serve to triumph.
Henin-Hardenne, who will now lose her place at the top of the world rankings, seemed to be finally getting her season back on track when she won Olympic gold in Athens last month.
But the 22-year-old Australian Open champion appeared jaded at Flushing Meadows and has clearly not fully recovered from a virus that laid her low for three months.
"Petrova played well, she served well and took her opportunities," said
"I didn't have confidence in my game. I wasn't feeling powerful and strong like in
the past. I was nervous. It just wasn't my day.
"I miss this kind of situation with a lot of pressure. In Athens everything was so different because there was less pressure. It was my coming back.
"I need more matches and more tournaments until the end of the
year to be ready for next season."
Petrova gathered confidence as the match went her way, helped by her accurate serves and powerful ground strokes.
The 14th seed faltered as she served out for the match, allowing Henin-Hardenne to grab a break point.
But she fired down a mighty serve to seal the match and book her place in the last eight.
"I am very satisfied," said Petrova. "I played great. My shots,
my serve was working. I was consistent and I am happy with myself."
The Russian, who had not beaten Henin-Hardenne in five previous encounters, meets compatriot Svetlana Kuznetsova for a place in the semi-finals.
Mary Pierce - conqueror of Wimbledon champion Maria Sharapova - was beaten 7-6 6-2 in the fourth round by the ninth seed.
The Frenchwoman matched Kuznetsova in the opener before losing a tight first-set tie-break.
But Kuznetsova stepped up a gear in the second set to reach the last eight in one hour and 23 minutes.