Agnieszka Radwanska of Poland celebrates her stunning victory over No. 2 seed Maria Sharapova of Russia on Saturday at the U.S. Open in New York.

New York ? Bothered by swirling gusts and a teen’s surprising moxie, Maria Sharapova was completely lost.

Agnieszka Radwanska of Poland pulled off the shocker, beating the defending U.S. Open champion 6-4, 1-6, 6-2 in a third-round match that took a startling turn Saturday.

Sharapova seemed to take control by winning eight games in a row, giving her a 2-0 lead in the final set. Then, she fell apart and never won again.

“A lot of girls in her position on a big stage in a big stadium against a top player sometimes get a little nervous,” Sharapova said. “She held on there.”

“You’d think I would have the momentum, but I just didn’t really feel that way inside of me for some reason.”

Sharapova became the first No. 2-seeded woman to lose before the round of 16 at the Open since Andrea Jaeger in 1981. Her father could barely stand to watch, moving from his front-row perch to a seat farther back during the final points.

“I could sit here and blame the wind and blame my opponent. But there are no excuses,” Sharapova said. “I’m not going to throw myself a pity party here.”

The 30th-ranked Radwanska had a lot to do with Sharapova’s undoing. At 18, the former junior champ at Wimbledon and the French Open played with poise and smarts, creeping closer and closer to the service box on second serves, hoping to unnerve the two-time Grand Slam winner.

“She was the favorite and I think she was more nervous,” Radwanska said.

At times, Radwanska walked nearly halfway from the baseline to the net as Sharapova got ready, then backed off at the last second.

“I knew that she hates if somebody is moving on the serve,” Radwanska said, her braces gleaming in the sunlight. “She made many mistakes.”

No such drama in the most-anticipated match of the tournament – Roger Federer was too much for 6-foot-9 rookie John Isner in a 6-7 (4), 6-2, 6-4, 6-2 victory.

Fifth-seeded Andy Roddick breezed, beating 2002 Australia Open champion Thomas Johansson of Sweden 6-3, 6-2, 6-0 to reach the fourth round.

In other men’s matches, No. 4 Nikolay Davydenko of Russia beat No. 28 Nicolas Almagro of Spain 7-5, 6-0, 7-5, while No. 9 Tomas Berdych of the Czech Republic and No. 10 Tommy Haas of Germany advanced.

Donald Young lost to Feliciano Lopez of Spain 3-6, 6-3, 7-6 (5), 7-5.