Top-seed Kim Clijsters [Images] tested the limits of her endurance with a 6-4, 3-6, 6-3 win over Jelena Jankovic to move into the quarter-finals of the Acura Classic in San Diego on Thursday.

Clijsters was joined by second seed Maria Sharapova [Images], who beat fellow Russian Vera Zvonareva 6-4, 6-4. Sharapova will face Mary Pierce in the quarters after the defending champion blitzed Finland's Emma Laine 6-2, 6-1.

Belgian Clijsters will face eight seed Martina Hingis [Images] in the last eight after the Swiss beat Flavia Pennetta 6-2, 3-6, 6-1.

Clijsters looked sluggish against the hard-hitting Jankovic, but held her nerve in the third set to see off her opponent.

"My legs felt dead today, but I just tried to hang in there," said Clijsters, who was playing her sixth match in eight days.

"I felt like it was so hard to push myself, but I tried. I was trying to find a way to get back to where I wanted my legs to be."

After letting three break points slip, Clijsters finally broke Jankovic to take a 4-3 lead in the third set.

"In the third set, it's a little of a mind game," said Clijsters, who has won 42 of her last 44 matches in North American hard court events.

"That's always going to have a little bit of an influence on both the person who wins and loses the game."

Hingis, who lost to Clijsters in the Australian and French Open quarter-finals this year, predicted the quarter-final would be a battle.

THE GRIND

"You have to commit to a long match and grind it out," Hingis said.

"You have to go in knowing in your head you have to rally with her. You cannot overpower her. You have to be able to run down a lot of shots and take every opportunity you can get."

Sharapova, playing in her first tournament since falling to Amelie Mauresmo in the semi-finals of Wimbledon, relied on a big serve and solid defense to see off Zvonareva.

After a close first set, Sharapova raced out to 4-0 lead in the second, only for the speedy Zvonareva to claw her way back to 4-5. But Sharapova's serve held firm and she closed the match out.

"Every time I play her, I expect a good test because I know how good and strong she can be," said Sharapova.

"It was about reacting to the situation and seeing what she brought today and what I needed to challenge her."

In other matches, fourth seed Elena Dementieva of Russia [Images] came back from a break down in the third set to beat India's Sania Mirza [Images] 6-2, 3-6, 6-3. She will face fifth seed Patty Schnyder, who beat Argentine Paola Suarez 6-3, 3-6, 6-1.

Czech seventh seed Nicole Vaidisova defeated Daniela Hantuchova [Images] 6-4, 6-2 and will play 19-year-old Russian Anna Chakvetadze [Images], who upset 14th seed Ana Ivanovic 4-6, 6-4, 6-1.

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