NEW YORK - Trailing big in the first round of the U.S. Open, Maria Sharapova thought - well, no, she was certain - that she'd pull through if she could push her inexperienced opponent to a third set.

And Sharapova was right.

Shrieking as loudly as ever, Sharapova came back from a set and a break down against 19-year-old Heather Watson of Britain to win 3-6, 7-5, 6-3 on Monday, improving to 12-0 this year in matches that went the distance.

"It's just a matter of belief within myself, that no matter how well or bad or good I'm playing, or my opponent is playing, I know I can tough it out," the No. 3-seeded Sharapova said after her 2?-hour victory. "No matter what the situation is, I have the belief."

That self-confidence comes not merely from her success in three-setters this season, but also from three Grand Slam titles, including the 2006 U.S. Open. It's the sort of track record the 102nd-ranked Watson hopes to have one day; Monday's match was only her fifth at a major tournament.

Sharapova won six Grand Slam matches at Wimbledon alone this summer, reaching the final there before losing to Petra Kvitova. Fresh off that triumph, Kvitova - a 21-year-old from the Czech Republic seeded No. 5 in Flushing Meadows - failed to follow it up, flopping at the U.S. Open with a 7-6 (3), 6-3 loss to 48th-ranked Alexandra Dulgheru of Romania.

Kvitova is the first reigning Wimbledon women's champion to lose her first match at the U.S. Open in the same season. Only three times had the Wimbledon winner bowed out as early as the third round in New York: Sharapova in 2004, Conchita Martinez in 1994, and Billie Jean King in 1973.

"This is something new for me," Kvitova said about her new status as Grand Slam champion. "I've felt a little pressure."

She was the only seeded woman to exit on Day 1 of the year's last major tournament, joined on the way out by No. 15 Viktor Troicki of Serbia, a 3-6, 6-3, 4-6, 7-5, 7-5 loser against Alejandro Falla of Colombia.

At night, 2000-01 U.S. Open champion Venus Williams played her first match in two months and beat 91st-ranked Vesna Dolonts of Russia 6-4, 6-3. Williams hit six aces and 28 total winners against the weary Dolonts, who left Moscow at 4 a.m. EDT and arrived at the Billie Jean King National Tennis Center at 4 p.m., after having flights canceled Saturday and Sunday because of Tropical Storm Irene.

"It's always nerve-racking to play the first match after a layoff in a major. It's not really my first choice at all," said Williams, who pulled out of recent tuneup tournaments because of a virus. "But I just tried to rely on experience and, I don't know, just tried to get after it. So I was pleased with the level."

In the day's last match, 16-time major winner Roger Federer beat 54th-ranked Santiago Giraldo of Colombia 6-4, 6-3, 6-2 to collect his 224th victory in Grand Slam play, tying Andre Agassi for second-most in the Open era.

"It doesn't need to be pretty. I don't think I've ever played my very, very best in the first round. It's always nice to be winning matches," said Federer, who now trails only Jimmy Connors' career total of 233.

Other winners included No. 7 Gael Monfils; No. 8 Mardy Fish, who played his first U.S. Open match as the top-seeded American and beat Tobias Kamke 6-2, 6-2, 6-1; No. 9 Tomas Berdych, the 2010 Wimbledon runner-up; No. 13 Richard Gasquet; No. 22 Alexandr Dolgopolov; and No. 27 Marin Cilic, who eliminated 19-year-old American Ryan Harrison 6-2, 7-5, 7-6 (6).

"This is extremely different for me," Fish said, "this feeling coming out here and trying to show everything you can, to show you're the No. 1 guy, at least for this tournament. It's been a lot of fun."

Advancing along with Sharapova to the second round were No. 2 Vera Zvonareva, a finalist last year at Wimbledon and the U.S. Open; 16-year-old Madison Keys of Boca Raton, Fla. - the youngest and, at 455th, lowest-ranked woman in the draw - who beat 37-year-old Jill Craybas 6-2, 6-4; and No. 12 Agnieszka Radwanska of Poland, who beat her younger sister Urszula Radwanska 6-2, 6-3.

But surprise 2009 U.S. Open quarterfinalist Melanie Oudin of Marietta, Ga., lost 6-0, 7-6 (7) to Romina Oprandi of Italy, falling to 9-29 in 2011.

Sharapova was one of the seeded players Oudin stunned during her run two years ago, and for a little more than a set Monday, Watson seemed quite capable of registering another significant surprise.

Scrambling along the baseline to get to nearly every ball, Watson forced Sharapova to hit extra shots in order to win a point. And Sharapova, who said she wasn't able to practice enough over the weekend because of Tropical Storm Irene, kept missing.

"There's no doubt that I wasn't playing my best tennis," said Sharapova, who finished with a whopping 58 unforced errors, nearly twice as many as Watson. "She was smart in making me hit another ball. I was making so many errors out there. She stuck to her game plan; she kept grinding."

After taking the first set, Watson broke for a 1-0 lead in the second. That's when Sharapova began to turn things around, taking four games in a row. Watson didn't go away, though, getting within 4-3 when Sharapova double-faulted, then holding for 4-all with the Arthur Ashe Stadium crowd supporting the underdog.

But a double-fault by Watson, and two huge return winners by Sharapova, helped the Russian break to end the second set. That sent the match to a third, and Sharapova's as good as it gets there.

"Maria's a fighter. She's never going to give up," said Watson, who got high-fives and autograph requests from fans as she left the court. "That's what makes her a champion. That's why she's won this tournament before."

Kvitova, in contrast, said that as she began to make mistakes, she started thinking negative thoughts. Asked why she was still struggling with that after winning Wimbledon, she said, "That's a good question, actually."