Facing her toughest opponent of the tournament, the top-seeded Williams relied on solid groundstrokes to defeat Molik in 1 hour, 33 minutes at Ariake Colosseum.

"I was having some problems with my errors which was frustrating," Williams said. "She played well and had the skills to win the match."

Williams struggled with her serve in the first set, double faulting five times, but took control midway through the second set as Molik made some costly errors.

"There were times when I was good and times when I wasn't," Williams said. "Now I just have to focus on the semifinals."

Williams, seeking her fourth title this season after winning the Korea Open on Sunday, went ahead 4-3 when Molik double faulted in the seventh game of the second set and then held serve to go up 5-3. She won the match with a crosscourt forehand off Molik's serve.

In other women's quarterfinal matches, Virginie Razzano defeated fellow Frenchwoman Camille Pin 6-2, 6-2, while eighth-seeded Flavia Pennetta of Italy defeated second-seeded Sania Mirza of India 6-4, 6-4.

In the men's tournament, top-seeded David Ferrer of Spain defeated Vincent Spadea of the United States 6-1, 7-6(0) and Croatia's Ivo Karlovic, seeded seventh, ground out a 7-6 (6), 6-7 (8), 7-6 (8) win over ninth-seeded Lee Hyung-taik of South Korea to advance to the quarterfinals.