Showing signs of shakiness in her first Grand Slam quarterfinal, the 17-year-old Oudin got off to a slow start against No. 9-seeded Caroline Wozniacki of Denmark and never really recovered, losing 6-2, 6-2 Wednesday night.
Oudin, who is ranked 70th, upset four more established players - including three-time major champion Maria Sharapova and Beijing Olympic gold medalist Elena Dementieva - to become the youngest quarterfinalist at Flushing Meadows since Serena Williams in 1999.
Making the story even better: Oudin's last three victories each came after dropping the first set. But Wednesday's start was quite inauspicious: She lost 14 of the first 18 points under the bright lights in the big city.
With "BELIEVE" stamped in all capital letters on the heels of her pink-and-yellow sneakers - and, up in the player guest box, her twin sister and coach wearing black T-shirts bearing that word, too - Oudin certainly never gave up. Her groundstrokes let her down, though.
Oudin made 43 unforced errors, 23 more than Wozniacki. A relative veteran by comparison, the 19-year-old Wozniacki leads the women's tour in match victories this season.
Now the Dane will play in her first Grand Slam semifinal against another teen, 19-year-old Yanina Wickmayer of Belgium. The 50th-ranked Wickmayer - never before past the second round at a Grand Slam tournament - reeled off the last five games to beat Kateryna Bondarenko of Ukraine 7-5, 6-4.
The other women's semifinal Friday features two far more familiar names: defending champion Serena Williams against 2005 champion Kim Clijsters.
To no one's surprise, Oudin did make bids to get back into her match against Wozniacki.
At 1-1 in the second set, Oudin held two break points - and pushed a forehand return of a 71 mph second serve wide, then sailed a forehand long.
Then, at 2-all, Oudin again earned two break points - and sent a backhand wide on the first, then a forehand long on the second.
And that, essentially, was that. Wozniacki won that game and each of the next four, ignoring a partisan crowd that cheered when she double-faulted.
In men's action Wednesday, No. 4-seeded Novak Djokovic reached the U.S. Open semifinals for the third consecutive year, beating No. 10 Fernando Verdasco of Spain 7-6 (2), 1-6, 7-5, 6-2.
Djokovic, the 2008 Australian Open champion, lost to Roger Federer in the 2007 final and the 2008 semifinals at Flushing Meadows.
He could meet Federer again this year: After the Oudin-Wozniacki match, Federer was to face No. 12 Robin Soderling in the quarterfinals, with the winner facing Djokovic.
While so much of the focus around these parts has been on Oudin, Wickmayer's story is quite intriguing and inspiring.
When she was 9, her mother died of cancer, and little Yanina set out to find a fresh start, researching tennis academies on the Internet before settling on one in Florida.
Talk about precocious, ambitious and adventurous: Yanina had only recently started playing tennis. Neither she nor her father spoke English.
But this is what had to be done.
Her father closed his pool construction company in Belgium, and relatives supported the pair financially while they lived in Florida for 2½ years.
"He just gave everything up for me," Yanina said. "He just left. He listened to a girl that was 9 years old and left his life, left his dreams. I'm always going to respect him for that."
Marc Wickmayer was in the Arthur Ashe Stadium stands Wednesday, watching his daughter play the biggest match of her career - and win it.
"I have no words for what he's done," Yanina said. "There is no way of thanking him in any way for what he did, but I hope with my semis here this week, I can show him that I really thank him for everything."