By Larry Fine

NEW YORK (Reuters) - Serena Williams struggled with her serve in the first set before finding the range to score a 7-5 6-1 victory over Anastasia Pavlyuchenkova of Russia on Thursday to reach the semi-finals of the U.S. Open.

Three-time winner Williams lost serve three times in the opening set as the first six games went against serve before the American got on track, breaking the 17th-seeded Russian in the 12th game to end the opening set.

"It was a slow start, and once I lost serve, I was determined to break serve so I wouldn't be down a break," said Williams. "Then I just kept losing serve in the beginning. Very weird."

The 29-year-old Williams found a way to get fired up.

"I tried to move my feet a little better and just tried to get pumped up, because I felt like I wasn't at my best. I just tried to do everything just a little bit better."

The formula proved good enough. Williams ended up striking 26 winners, double the number posted by Pavlyuchenkova, who struggled to win points on her weak second serve.

Williams, who won 22 of 26 points on the Russian's second serve, got on a roll in the next set, losing just three points on serve and ending the 79-minute match with her sixth ace.

The victory lifted Williams into the last four against top-seeded world number Caroline Wozniacki of Denmark, a 6-1 7-6 winner against 10th-seeded German Andrea Petkovic.

Wozniacki will play 13-time grand slam winner Williams on Saturday with the winner going onto the final, which will be played on the 10th anniversary of the September 11, 2001, attacks.

"I think everyone that lives in America ... has been affected by 9/11," said Williams, who is playing in just her sixth tournament this year due to injuries and illness. "I was in (Washington) D.C. at the time, and I just remember seeing a lot of Army trucks.

"It's hard to believe it's 10 years later, but it's good. Good we are kind of coming together, and New Yorkers and New York has been so strong."

(Editing by Frank Pingue)