NEW YORK (AFP) - Wimbledon winner Petra Kvitova crumbled in the first round of the US Open on Sunday, as Maria Sharapova showed her championship mettle with a battling victory. Kvitova tumbled out of the tournament at the hands of unheralded Romanian Alexandra Dulgheru while Sharapova who she beat for the Wimbledon crown eked out a three-set win over British teenager Heather Watson. On the mens side, third-seeded Roger Federer secured a 6-4, 6-3, 6-2 victory over Colombian Santiago Giraldo, although he was far from his fluent best. Kvitova, ranked seventh in the world and seeded fifth, made 52 unforced errors in a 7-6 (7/3), 6-3 defeat to 48th-ranked Dulgheru. Meanwhile Sharapova battled back from a set and a break down to beat Watson 3-6, 7-5, 6-3. It was another post-Wimbledon disappointment for Kvitova, the 21-year-old Czech who has been unable to build on her Grand Slam breakthrough. Its something new for me, Kvitova said of the pressure of carrying the title of Grand Slam champion into a tournament. Everything is in the head, she added. If you are thinking on court negatively its bad. In an erratic showing, Sharapova made 58 unforced errors, but notched 41 winners helping her remain unbeaten in a dozen three-set matches this season. The best thing about this match is I gave myself a chance to play another one, Sharapova said. Federer did the same, and while he wasnt at his sharpest in the early going the outcome was never in doubt against the 54th-ranked Giraldo, who has failed to beat a top-10 player. Couldnt be better, the five-time champion said. It was a great atmosphere, great ovation when I walked out. I really enjoyed it. Federer acknowledged that it wasnt his best tennis, but said it didnt have to be. It doesnt need to be pretty, Federer said. I dont think Ive ever played my very, very best in the first round. Federer will no doubt need to be more convincing over the coming days at Flushing Meadows, where he is seeded behind world number one Novak Djokovic and defending champion Rafael Nadal. A five-time US Open champion, Federer won the most recent of his 16 Grand Slam titles at the 2010 Australian Open, and he needs a triumph here to extend his streak of eight consecutive seasons with at least one Grand Slam crown. He started slowly under the lights on the Arthur Ashe stadium, with 17 of his 35 unforced errors coming in the opening set. As the match went on, I think I started to get more solid and better and thats a good feeling to have, Federer said, adding that he thought the Flushing Meadows courts were playing slower this year than in the past. Mardy Fish, playing his first Grand Slam as the top-ranked American, opened the action at the Arthur Ashe stadium, where he beat Germanys Tobias Kamke 6-2, 6-2, 6-1. This is extremely different for me, this feeling coming out here and trying to show everything you can, to show youre the number one guy, at least for this tournament, the eighth-seeded Fish said. Fish next faces Malek Jaziri, a qualifier ranked 184th in the world who became the first Tunisian to win a Grand Slam match when he beat Dutchman Thimeo de Bakker 4-6, 6-1, 6-4, 6-2. Colombian Alejandro Falla, ranked 119th in the world, survived three match points to topple 15th-seeded Serbian Viktor Troicki 3-6, 6-3, 4-6, 7-5, 7-5. Frances Gael Monfils, the seventh seed, advanced with a hard-fought 7-6 (7/4), 6-3, 6-4 victory over Grigor Dimitrov. Serenas sister Venus Williams, who is unseeded after a season spent largely on the sidelines, advanced with a 6-4, 6-3 victory over Russian Vesna Dolonts. Williams, winner of back-to-back US Opens in 2000 and 2001, was playing her first match since Wimbledon and just her fourth tournament of the year. Its always nerve-wracking to play the first match after a layoff in a major, said Williams, who grinned and clenched her fist after firing a service winner past Dolonts on match point. But I just tried to rely on experience and just tried to get after it. Other seeded women who advanced included second-seeded Russian Vera Zvonareva, a finalist here last year, eighth-seeded Marion Bartoli of France and ninth-seeded Australian Samantha Stosur. Zvonareva beat Stephanie Foretz-Gacon 6-3, 6-0. Bartoli beat Russian Alexandra Panova 7-5, 6-3, and Stosur beat Swedens Sofia Arvidsson 6-2, 6-3. Chinas 13th-seeded Peng Shuai defeated American Varvara Lepchenko 6-3, 6-4 and Radwanska beat Urszula Radwanska.