Wimbledon champion Maria Sharapova [Images] beat former world number one Justine Henin-Hardenne 6-1, 6-7, 6-2 in the quarter-finals of the Nasdaq-100 Open on Tuesday to end the Belgian's comeback run.

Sharapova will play either Venus or Serena Williams [Images] in the semi-finals. The sisters play each other later on Tuesday evening.

In the men's draw, six-times champion Andre Agassi [Images] went through to the last eight after beating French Open champion Gaston Gaudio 7-6, 6-2.

"A match like that can just turn into a first class battle or if you take your right chances, you can maybe break a match like that open," said Agassi, who will play either Taylor Dent or Florian Mayer in the quarter-finals.

"Today, that's what happened."

Top seed Roger Federer [Images] survived a stiff fourth round test from Mario Ancic, eventually beating the Croatian 6-3, 4-6, 6-4 to record his 29th win of the season.

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The Swiss has lost just one match since last August, to Russian Marat Safin [Images] in January's Australian Open semi-finals.

After whipping through the first set in 27 minutes, Sharapova had chances to win far more emphatically.

The 17-year-old Russian served for the match at 5-4 in the second set and held three match-points only to be pegged back.

After the Belgian had taken the tiebreak to level the match, Sharapova received treatment on her lower back.

"I've had a back problem for a little while already," Sharapova said. "I did so much running today that it started to get stiff. I got some treatment and I just tried to forget about it. Hopefully I'll be back fresh."

The treatment seemed to help Sharapova and she broke Henin's serve to go 2-0 up in the third and never allowed her advantage to slip.

BACKHAND WINNER

After two hours and 17 minutes she took the match with a backhand winner.

"Things can turn around quickly, that's tennis for you," said Sharapova. "I knew I had to lift my game in the third set because all of a sudden she was getting a lot more balls back. She's a huge fighter out there."

Henin was playing her first tournament since last year's U.S. Open.

The Belgian spent much of last season battling the virus mononucleosis and was sidelined for the first three months of the 2005 season after she injured her knee while preparing for the Australian Open in January.

In other men's fourth round action, Dominik Hrbaty beat French teenager Gael Monfils 6-3, 6-3 and will play David Ferrer in the quarter-finals. The Spaniard defeated countryman Juan Carlos Ferrero 6-7, 6-3, 7-5.

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