NEW YORK — It's not quite the case that 17-year-old Melanie Oudin and her family knew for sure she would get this far, this fast.

Not when Melanie was 7, hitting buckets of tennis balls with Grandma Mimi back home in Marietta, Ga. Not a couple of years later, when Melanie and her twin sister began taking lessons together. And certainly not when Melanie lost her first two Grand Slam matches.

Still, there was Oudin at the U.S. Open yesterday, ranked all of 70th, dealing with a painful leg and an overwhelming occasion on a supersized stage — and stunning No. 4-seeded Elena Dementieva, 5-7, 6-4, 6-3, to set up a third-round match against 2006 champion Maria Sharapova.

All the while, Oudin sported this word stamped near the heel of her pink-and-yellow sneakers: "BELIEVE." The idea for that bit of inspiration came from her boyfriend, Austin Smith, a 15-year-old who helped Melanie prepare for her Arthur Ashe Stadium debut by practicing together in the 23,763-seat arena at 9:30 a.m. yesterday.

"During the match, I had confidence, and, I mean, I was right there with her the entire time," Oudin said. "She wasn't blowing me off the court. She wasn't hitting winners left and right on me."

Don't, though, get the mistaken idea that Dementieva played poorly or showed the sort of nerves she has in the past. Dementieva played rather well — displaying the stinging groundstrokes that carried her to two Grand Slam finals and an Olympic gold medal — and graciously gave credit to Oudin, who now will face the scrutiny that comes with being the "Next Great American Hope."

After the Williams sisters at No. 2 and 3, you have to scan all the way down to Oudin to find the next U.S. woman in the WTA rankings.

"It's just the beginning," Dementieva cautioned, "but it looks like she has a good future."

Truth is, Oudin — pronounced "oo-DAN," owing to her father's French ancestry — has a pretty good present, too. This was not, after all, her first such upset at a major tournament: Oudin reached the fourth round at Wimbledon by beating former No. 1 Jelena Jankovic.

"She knows," said Brian de Villiers, Oudin's coach, "that she can play with these girls now."

Next comes what figures to be a stern test against three-time major champion Sharapova, who eliminated another 17-year-old American, Christina McHale of Englewood Cliffs, N.J., 6-2, 6-1, at night. The 381st-ranked McHale was a wild-card entry who didn't really challenge Sharapova.

Looking ahead to facing Oudin, Sharapova said: "I mean, I've got a tough round ahead of me. Also somebody that I've never played against before, someone that's going to come out and, I'm sure, she's going to swing and have nothing to lose — which she doesn't."

Jankovic made another early departure from a Grand Slam event, losing to 55th-ranked Yaroslava Shvedova of Kazakhstan, 6-3, 6-7 (4), 7-6 (6). Jankovic reached the U.S. Open final in 2008, and she was seeded No. 5 this year, but her head might not have been focused on the court on this day: Her grandmother died Wednesday night.

The losses by Dementieva and Jankovic mean half of the top 20 seeded women are out of the draw.

No. 1 Dinara Safina nearly joined the parade of surprising exits, turning in her second poor performance of the week before hanging on to edge 67th-ranked Kristina Barrois of Germany, 6-7 (5), 6-2, 6-3.

No seeded men lost yesterday, when the winners included 2007 runner-up Novak Djokovic and five Americans: No. 5 Andy Roddick, No. 21 James Blake, No. 22 Sam Querrey, 55th-ranked John Isner and 276th-ranked Jesse Witten.

Roddick, the 2003 U.S. Open champion and 2006 runner-up, beat Marc Gicquel of France, 6-1, 6-4, 6-4, at night and now takes on the 6-foot-9 Isner.