Russia's Elena Dementieva reached the US Open semi-finals with a hard-fought 4-6 6-4 7-6 win over Amelie Mauresmo.
The pair swapped breaks in the opening spell and the Frenchwoman had to claw back from 0-40 down to hold serve in the 10th game to take the first set.
The pattern continued in the second but this time the Russian, struggling with a thigh injury, dug deep to level.
The decider swung in both players' favour but Dementieva took the tie-break 7-1 to upset the second seed.
"I was dying for every point out there," Dementieva said. "I am very happy.
"It was tough to win especially with an injury."
Mauresmo's loss saw her miss a chance to assume the world number one ranking from Justin Henin-Hardenne.
She can still claim top spot but American Lindsay Davenport can overtake her if she wins the US Open crown.
"I want to achieve a Grand Slam victory, I just hope it's going to happen very soon," said a disappointed Mauresmo.
"It would be great to be number one but I wanted to do it
in a good way.
"I'm a bit angry with myself because I didn't take my opportunities."
Mauresmo appeared to be in control in the first set, taking the decisive break in the ninth game.
But after Dementieva had some lengthy treatment the second seed handed the Russian triple break point.
The Frenchwoman was forced to grit her teeth and produced some fierce backhands and an ace to seal the set.
Dementieva appeared to be rejuvenated by both her injury and falling a set behind.
The 22-year-old sent some forehands rasping down the court to break Mauresmo at 5-4 and held serve to force the decider.
Dementieva quickly went 2-0 up but it did not last as Mauresmo took advantage of her weak serves to break back.
The pair traded breaks again before the Russian sixth seed again called her trainer on to treat a stomach problem at 5-4 up.
Mauresmo and Dementieva, now apparently untroubled by physical problems, raised their levels again to take the match to a tie-break.
The Russian powered down some lashing forehands, as Mauresmo's faltered, to race to a 7-1 victory and seal the match.