NEW YORK—Melanie Oudin posted her second major upset in a Grand Slam this season, stunning No. 4- seeded Elena Dementieva, 5-7, 6-4, 6-3, on Thursday to reach the third round of the U. S. Open.

Earlier this year, the 17-year-old from Marietta, Ga., ousted former No. 1 Jelena Jankovic in the third round at Wimbledon.

“During the match, I had confidence, and, I mean, I was right there with [Dementieva] the entire time,” Oudin said.

Next for Oudin comes what figures to be a stern test against three-time major champion Maria Sharapova, who eliminated another 17-year-old American, Christina McHale of Englewood Cliffs, N. J., 6-2, 6-1.

Meanwhile, Jankovic made another early departure from a Grand Slam, 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.

No seeded men lost in early action Thursday, when the winners included 2007 runner-up Novak Djokovic and No. 21 seed James Blake.