Mary Pierce put a dent in Kim Clijsters's [Images] pursuit of the number one ranking with a 6-1, 4-6, 7-6 victory over the Belgian in the opening group match of the season-ending WTA Championships on Tuesday.

Top ranked Lindsay Davenport [Images] experienced no such problems and beat Russia's [Images] Nadia Petrova 6-2, 7-6 in her first round-robin match in the eight-woman draw, while champion Maria Sharapova [Images] prevailed 6-1, 3-6, 6-3 over Switzerland's [Images] Patty Schnyder.

Clijsters, who is not defending any points after skipping last year's event with a wrist injury, lies just 155 points behind Davenport in standing and was hoping a good run in California would help her to dislodge the American from the top.

However, in an extraordinary effort, Frenchwoman Pierce came from a break down on three occasions in the deciding set to beat the world number two for the first time in four encounters.

"I felt really tired," Clijsters said. "It's just a matter of jet lag. It's disappointing.

The only thing I want to do is go to the hotel and sleep. I was not seeing the ball at all. Even with the lights I felt it was just too bright out there.

"It was hurting my eyes and when I was trying to focus. It was really tough. I guess it's part of the long season I had with all the matches and travelling."

Looking rusty and far from the player who had thrashed Pierce in the U.S. Open final two months ago, Clijsters got off a poor start, failing to gain control of her forehand and frequently double-faulting.

AGRESSIVE TENNIS

Pierce deployed an extremely aggressive brand of tennis and powered an array of winners into the corners to cruise through the opening set.

Clijsters refused to be bowed, however, and regained her composure in the second set, stretching her 30-year-old opponent out in long rallies and not allowing Pierce to dictate points with her serve or return.

Buoyed after levelling the match, Clijsters appeared to be on her way to victory in the third set when she broke Pierce in the opening game, but the Frenchwoman kept charging while the Belgian struggled to find consistency with her serve or forehand.

Clijsters broke for a third time in the decider to take a 6-5 lead when the Frenchwoman missed a forehand long, but was unable to serve out the match and Pierce broke back when she belted an overhead into the Belgian's body.

Pierce carried that momentum into the tiebreaker, cracking five winners to set up four match points and sealing victory when a Clijsters backhand sailed long.

The 22-year-old Clijsters committed 31 unforced errors to only 21 from Pierce and double-faulted 11 times, leaving her likely to need victories over Amelie Mauresmo and Elena Dementieva in the Black Group to reach the semi-finals.

Playing her first match in three weeks after struggling with a right pectoral muscle injury and a right thumb problem, Sharapova looked rusty at times but turned up the power when she needed to.

"It was up and down," Sharapova said after her Green Group victory.

"I didn't really think I would play my best today. I felt a little awkward in some situations because I haven't been match tough lately.

"But I think I will get better. This is the last tournament of the year, so I better get it together quickly."

On Wednesday, Pierce will face Dementieva and Mauresmo will play Clijsters in the Black Group, while Davenport will take on Schnyder in the Green Group.

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