Williams, who had strained her right hamstring in practice, defeated Australia's Nicole Pratt 6-3, 3-6, 6-4, to book a second-round date with Bulgarian qualifier Pavlina Stoyanova.

"I don't think I played that well," Williams said. "My game is off. I am going to have to play better whether I want to or not."

Serena Williams, 16, and ninth seed Irinia Spirlea, who touched off a controversy by bumping with Venus in last year's semifinals, are on a collision course of their own for a third-round matchup.

"I'm not looking to that," Serena Williams said. "I'm just looking one match at a time. Usually in all the tournaments I look at the draw and I haven't won any. So let me try something different."

Williams surrendered a break with a backhand into the net to trail the third set 4-3 but rallied, breaking back and winning with another on her third match point.

Pratt sent a forehand into the net after Williams hit a forehand wide and a backhand into the next on her earlier closing chances.

"It was pretty exciting," Williams said of her U.S. Open debut. "I didn't want to go out in the first round. It just would have been horrible."

Venus, 18, was watching in the stands along with their father-manager Richard Williams, who walked out in the second set.

"If I was in the crowd and I saw someone playing the way I was, I would have left too," Serena Williams said.

Williams said she would have liked more than one tournament after Wimbledon to better prepare for the U.S. Open but schoolwork got in the way. Serena has environmental science and economics tests to take before completing high school.