Perebiynis saved three match points before Williams won with a powerful backhand.

"I don't I think I've played up to my standards today, but I was glad to get through," Williams said.

Dinara Safina also reached the quarterfinals, upsetting defending champion Anna Chakvetadze 7-6 (6), 6-2.

In the second round of the men's tournament, second-seeded Mikhail Youzhny was upset by Michael Berrer 7-5, 6-4, and Janko Tipsarevic rallied to beat third-seeded Andy Murray 6-4, 7-5.

Williams and Perebiynis traded breaks three times in the first set before the fourth-seeded American broke again with a backhand down the line in the 12th game.

The seventh-ranked Williams was leading 2-0 in the second set but lost her next serve at love and Perebiynis leveled at 3-3 after trading serves one more time.

Williams, who said she wasn't used to playing in morning matches, had trouble with her volleys early but managed to score several key points at the net in the second. Her first serve also improved, and she finished with six aces.

"Usually I sleep at 11, but I liked it because if I have to play hopefully four days in a row," Williams said.

In Friday's quarterfinals, Williams will play eighth-seeded Nicole Vaidisova , who beat Elena Vesnina 6-3, 6-4.

Williams, an eight-time Grand Slam champion, is playing in Moscow for the first time in a decade. In 1997, the 16-year-old qualified and lost in the first round when she was ranked 448th.

"She is a great champion and she's always tough," Vaidisova said. "I know it's going to be a very tough match tomorrow. She's always trying to her best."

After winning the first set, Tipsarevic struggled early in the second. He trailed 5-2, but then won five straight games to beat Murray.