Defending champion Maria Sharapova had been so dominant at the Open going into her third-round encounter with Agnieszka Radwanska of Poland that the talk surrounding her matches had been focused as much on her glittering outfits as it was on her game. Entering today’s match, she had lost only two games in two matches, both of which took less than an hour of work on court.

But the unheralded 18-year-old from Poland ended all talk of Sharapova’s on-court fashion, not to mention of her quest for a second US Open title. Radwanska, who was not overwhelmed by Sharapova’s power and played consistent, crafty tennis, took out the 2006 US Open champion and No. 2 seed, 6-4, 1-6, 6-2.

Radwanska, ranked No. 32 in the world, never buckled under the weight of playing the defending champ and revealed a fearlessness out on court, even at times moving in close to the service line when returning Sharapova's shaky second serve to try and throw off her higher-seeded opponent.

“I had nothing to lose," she said after the match. “Just she was the favorite, and I think she was more nervous. I just played my game.’’

Sharapova looked to be the far more powerful player throughout the match. But along with her 36 winners, she hit 49 unforced errors and 12 double faults, many of those doubles happening at inopportune times. Leading the first set, 2-0, Sharapova double faulted at break point to let Radwanska back in the match. She also double faulted while serving at 4-5, 30-all, giving Radwanska her first set point, which she capitalized on with a backhand passing shot winner up the line off a tentative approach to net by Sharapova.

Sharapova, who in this match in particular revealed in spades the lack of subtlety in her game, did get back on track in the second set, where she broke Radwanska three times and hit 14 winners to just 10 unforced errors, the only set in the match where she hit more winners than unforced errors. But after racing out to a 2-0 lead in the third, having won eight games in a row, Sharapova came undone. Radwanska won the last six games of the match, helped in the set by Sharapova’s 20 unforced errors, to pull off the biggest win of her career.

Sharapova did not have an answer after the match for how she lost the momentum she had going into the third set.

“You know, you’d think I’d have the momentum in the third with a break,’’ she said. “But I just didn’t really feel that way inside of me for some reason today. You know, just going to have those kind of days where you’re always trying to come through and always trying to find a way, even though you don’t feel like things are exactly going your way.’’

Sharapova knew she was in for a much tougher test today than she had faced in the tournament, although perhaps not quite as intense a challenge as Radwanska provided. They had played in Instanbul in May, and Sharapova needed three sets to win, although it was also played on clay, a surface that better suits Radwanska’s game. Sharapova was impressed with her then and even more so today.

“You got to give her a lot of credit for finishing the match and winning it,’’ she said. “A lot of girls in her position on a big stage in a big stadium against a top player sometimes get a little nervous. She held on there, and all the credit to her for winning the match.’’

This was the earliest Grand Slam loss for Sharapova since losing in the third round of the 2004 US Open. It also marks her worst showing at a Grand Slam this year. She lost in the final of the Australian Open to Serena Williams, the semifinals of the French Open to Ana Ivanovic, and the fourth round of Wimbledon to Venus Williams.

“Clearly it hasn’t been my year, just hasn’t been my year,’’ she said. “But that’s the way things go. I got to take the good, and I got to take the bad, and I got to move on.’’

For Radwanska, she’s moving on to celebrate the biggest win of her career.

“Before the match I spoke with my sister and said, 'If I beat [Sharapova], we will buy two Louis Vuitton bags,’ because it’s so expensive. And she was, ‘Okay, if you do it. O.K.’ Now, we are going to buy two bags.’’

Radwanska next takes on the winner of the Nicole Vaidisova-Shahar Peer match.