New York ? Trailing big in the first round of the U.S. Open, Maria Sharapova thought — 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 21/2-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.”

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 3 a.m. CST and arrived at the Billie Jean King National Tennis Center at 3 p.m., after having flights canceled Saturday and Sunday because of Tropical Storm Irene.

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 and 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.

“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.

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.