Serena Williams overcame a difficult first set to advance to the U.S. Open semifinals Thursday with a 7-5, 6-1 victory over Anastasia Pavlyuchenkova.

Williams, the 28th seed but the favorite to win at Flushing Meadows, lost serve in the first game of the match but won it back in a love game to make it 3-3, then got another break to win the first set 7-5.

The second set against the 17th-seeded Pavlyuchenkova took a half hour, and Williams moved two wins away from her 14th Grand Slam title.

“I just kept losing serve in the beginning. Very weird,” said Williams, whose older sister Venus watched from the stands.

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Top-seeded Caroline Wozniacki defeated No. 10 Andrea Petkovic 6-1, 7-6 (5) Thursday to set up a semifinal matchup against Williams.

Wozniacki let a 5-3 lead slip in the second set and wound up in a tiebreaker, but Petkovic made five unforced errors in the breaker and couldn’t complete the comeback.

Wozniacki, who has been ranked No. 1 for most of the last year, has yet to win a Grand Slam title. She is seeking a spot in her second Grand Slam final, but will have to get past Williams to do it.

The top-seeded player was put out on Court 13 after the court inside Louis Armstrong Stadium was deemed unplayable. Wozniacki followed Andy Roddick onto the outside court, which seats fewer than 600 spectators.