The final Grand Slam tennis event of the year, the U.S. Open, continues in New York Tuesday after an opening day that saw a major upset, close calls and wins by some of the favorites.

In the shock of the day, Wimbledon women's champion and fifth seed Petra Kvitova of the Czech Republic fell to 48th-ranked Alexandra Dulgheru of Romania, 7-6, 6-3. She is the first reigning Wimbledon women's champion to lose her opening match at the U.S. Open in the same season.

Third-seeded Russian Maria Sharapova had a close call, battling back from a first set loss to beat Britain's Heather Watson, 3-6, 7-5, 6-3. Other top-10 women's seeds advancing included number-two Vera Zvonareva of Russia and eighth-seeded Frenchwoman Marion Bartoli.

Unseeded former champion Venus Williams of the United States won her opening match in her first appearance since Wimbledon in June.

Among the highlights in men's play, five-time champion and third seed Roger Federer of Switzerland overcame an inconsistent start to defeat Colombia's Santiago Giraldo in straight sets .

With the win, the record-holding 16-time major winner tied American Andre Agassi for the second most grand slam singles victories with 224, nine behind Jimmy Connors of the United States.

In other men's action, number-seven Frenchman Gael Monfils, eighth-seeded and highest ranked American Mardy Fish and number-nine Czech Tomas Berdych all advanced to the second round. Serbia's Viktor Troicki was the only seeded player to lose.