After a rollercoaster start to her US Open quarterfinal encounter with No. 17 seed Anastasia Pavlyuchenkova on Arthur Ashe Stadium, three-time champion Serena Williams found her groove, defeating the 20-year-old Russian, 7-5, 6-2, to reach her sixth semifinal at Flushing Meadows Thursday afternoon.

"I think for me 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. [It was] very weird... she [was] definitely different. She'll hit hard, then she'll hit soft. She's great at mixing things up."

Both players struggled to find a rhythm for the first half of the opening set. Through six games, neither player held serve, with Pavlychenkova failing to win a single point on her second serve. The 28th-seeded Williams ended the streak in the next game with a love hold to lead for the first time, 4-3. The Russian followed suit, holding after Williams netted a backhand.

Their level of play picked up from there, but the American was the more consistent of the two. Down 5-6, Pavlychenkova faced set point after Williams crushed a crosscourt backhand winner. Not to be outdone, the first-time US Open quarterfinalist struck a crafty crosscourt backhand that clipped the line to get to deuce, and reached game point, before Williams won three straight points to take the first set in 49 minutes.

In the second set, Williams dominated the Russian, converting two breaks of serve to move ahead 5-1. A well played forehand volley set up two match points, and the American closed out the match with an ace out wide to claim the victory in 75 minutes. "I just think I was just a step slow in the beginning," admitted Williams. "I was hitting a lot indoors and it's a totally different surface, so once I kinda got out in the match, once I showed up, I was able to play better."

Williams improved to 2-0 against Pavlychenkova after defeating the former US Open junior champion in a three-set, third-round match at the French Open in 2010.

The 29-year-old will play world No. 1 Caroline Wozniacki in the semifinals. The 2009 US Open runner-up topped 10th-seeded Andrea Petkovic 6-1, 7-6(5) to advance to her second major semifinal of the season, having progressed to the final four at the Australian Open (l. to Li Na).

Williams holds a 2-0 edge against the Dane. The two last met in a semifinal clash at the 2009 WTA Tour Championships, which Williams took 6-4, 0-1 after Wozniacki retired with a stomach strain midway through the match.