CINCINNATI: Kim Clijsters kept her comeback rolling, but America's Williams sisters fell by the wayside at the two million-dollar WTA tournament.

Former world number one Clijsters, playing her first tournament in two years, booked a quarter-final berth with a 6-4, 4-6, 6-2 victory over sixth-seeded French Open champion Svetlana Kuznetsova.

The Belgian earned a shot at world number one-ranked Russian Dinara Safina, who defeated China's Peng Shuai 6-3, 6-4.

"I feel really good after this match," Clijsters said. "It's not that I feel tired or drained or anything, just a few little aches and pains from getting back into playing matches."

But as Clijsters' dream return to the courts continued, the day proved a nightmare for both Serena and Venus Williams.

Second-seeded Serena, the reigning US Open, Australian Open and Wimbledon champion, was stunned by 29th-ranked Austrian Sybille Bammer 7-5, 6-4.

Red-hot Italian Flavia Penetta, fresh from victory in Los Angeles on Sunday, toppled third-seeded Venus Williams 7-6 (7/2), 6-4.

Bammer broke the error-prone Serena three times en route to the victory.

"I never played worse in my life," Serena said. "It was really a bad match."

"I just tried to come into the rallies and make her play, and this worked," Bammer said.

Serena took the court after Venus was beaten, and was hoping to get at least one Williams into the quarters.

"We don't lose too much in the same day, so usually I'm so pumped," Serena said. "And I was pumped today. I just couldn't get it together."

Nothing was coming together for Venus, whose 38 unforced errors included untimely forehands long in the first-set tiebreaker and a forehand over the baseline on match point.

"I wasn't able to get control of my ball today," Williams said. "After a while, it was obvious to see she was just keeping the ball in play and waiting for me to self-destruct."

Pennetta will play Slovakian Daniela Hantuchova, who advanced with a 7-6 (8/6), 0-6, 7-6 (7/5) victory over seventh-seeded Vera Zvonareva of Russia.

Fourth-seeded Russian Elena Dementieva, who nabbed Olympic gold in Beijing last year, beat Romanian Sorana Cirstea 6-4, 6-4 to set-up a quarterfinal with rising Danish star Caroline Wozniacki.

Wozniacki the eighth seed, advanced as Hungary's Melinda Czink retired injured while trailing 3-0 in the opening set.

Bammer will play fifth-seeded Serbian Jana Jankovic, who beat Victoria Azarenka of Belarus 7-5, 7-6 (7/4).