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Wimbledon champion Petra Kvitova was the big name to fall on the opening day at the US Open in New York on Monday.

The Czech won her first Grand Slam title in July but has struggled for form since and went down 7-6 (7-3) 6-3 to Romania's Alexandra Dulgheru.

Second seed Vera Zvonareva and third seed Maria Sharapova both made it through to the second round.

Two-time champion Venus Williams, unseeded this year, beat Russian Vesna Dolonts 6-4 6-3 in the night session.

Playing her first match in two months because of injury, Williams made a slow start but went on to hit six aces and 28 winners in an encouraging display.

Dolonts had travelled from Moscow and only arrived at the tournament site about four hours before her match, after flights were cancelled on Saturday and Sunday because of Hurricane Irene.

"It's always nerve-wracking to play the first match after a lay-off in a major," said Williams.

"It's not really my first choice at all, but I just tried to rely on experience and just tried to get after it, so I was pleased with the level."

Fifth seed Kvitova won just one round in both Toronto and Cincinnati coming into the US Open and the 21-year-old admitted that the pressure of being a Grand Slam champion had affected her, saying: "It's something new for me."

She made 52 unforced errors against the unseeded Dulgheru and left the Louis Armstrong court almost sobbing as her opponent, ranked 49th in the world, soaked up the acclaim.

"Everything is in the head," said Kvitova. "If you are thinking on court negatively, it's bad."

Zvonareva, last year's runner-up, eased past qualifier Stephanie Foretz Gacon of France 6-3 6-0, and later denied that her relatively low profile bothered her.

"That's probably the last thought on my mind," said the Russian.

"I want to go out there and play deep into the tournament because it is a Grand Slam, because it is prestigious, because we've got the best field in the world here."]

Sharapova was made to battle for over two-and-a-half hours on Arthur Ashe Stadium before she beat Britain's Heather Watson 3-6 7-5 6-3.

Eighth seed Marion Bartoli saw off Russia's Alexandra Panova 7-5 6-3, and ninth seed Samantha Stosur beat Sofia Arvidsson 6-2 6-3.

Polish 12th seed Agnieszka Radwanska came through one of the more intriguing first-round matches as she defeated her sister, Urszula, 6-2 6-3.

"It's so hard to play a serious match," said Agnieszka. "We're best friends off the court."