Former world number one Kim Clijsters of Belgium was knocked out of the women's singles quarterfinals at the Wimbledon Championships on Tuesday.

The eighth seed Clijsters managed to win the opening set 6-4, but lost the following two sets to the 21st seed Vera Zvonareva from Russia 4-6, 2-6.

"Disappointed," said Clijsters who showed up at the grass-court Grand Slam tournament after a three-year absence from the All England Club. "It's too bad I wasn't able to come up with my best at the important time in the match."

Clijsters came from behind to edge her compatriot Justine Henin and reached the quarterfinals on Monday. While facing Zvonareva on Tuesday, Clijsters said she was kept under pressure by her opponent, with whom she held a 5-0 head-to-head record.

"She hits the ball very hard, flat and deep. It was up to me to try to go for the lines, open up the court a little bit more, especially to her forehand. I was trying to pull her out of the court a little bit. But she was really on top of my game I felt today," said Clijsters.

Clijsters, 27, announced her retirement in May 2007 but declared her return almost two years later.

In only her third tournament back, she won her second US Open title, becoming the first unranked player and wildcard to win the tournament.

"I think I'm more focused than I used to be," said Clijsters. "This is my first time back at Wimbledon. I haven't been back for a year yet. Obviously it takes time."

Zvonareva is going to fight against Bulgarian Tsvetana Pironkova, who ousted American second seed Venus Williams 6-2, 6-3 earlier on Tuesday.

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