MADRID - World number two Maria Sharapova racked up her 18th win in a row with a 6-4 6-4 victory over Kim Clijsters in the round robin stage of the season-ending WTA Championships on Wednesday.

Maria Sharapova of Russia returns the ball to Kim Clijsters of Belgium during their WTA Championships tennis tournament match in Madrid November 8, 2006. [Reuters]

Martina Hingis of Switzerland celebrates her victory over Nadia Petrova of Russia after their WTA Championships tennis tournament match in Madrid November 8, 2006. [Reuters]

The 19-year-old US Open champion tops the red group after two straight sets victories and has one foot in the semi-finals of a tournament she won in 2004.

"I'm definitely playing some really good tennis," Sharapova told reporters. "I know I've played a lot of matches and I feel good about my game."

Swiss Martina Hingis, who won the WTA Championships in 1998 and 2000 before injury problems set in, bounced back after her opening day defeat to Henin-Hardenne with a 6-4 3-6 6-3 victory over Russia's Nadia Petrova in the yellow group.

Hingis's win means that Henin-Hardenne can finish no lower than third in the group and will therefore garner enough points to stop Amelie Mauresmo holding on to the number one spot at the end of the year.

Sharapova is the only player who can now stop the Belgian becoming the year-end number one although she will have to reach the final to have a chance of claiming the honour.

Svetlana Kuznetsova, ranked four in the world, got off to a winning start with a 7-5 6-3 victory over Elena Dementieva in the all-Russian clash and moves into second place behind Sharapova in the red group.

Former world number one Clijsters made an impressive return to action after a two-month lay-off with a wrist injury by winning in Hasselt at the weekend, but there was little the Belgian could do to counter Sharapova's power play at the Madrid Arena.

UNDER PRESSURE

Backed by her heavy serve and pin-point ground strokes, Sharapova kept Clijsters under pressure to take the opening set 6-4.

The teenager turned the screw to break once again in the fifth and seventh games of the second set, but Clijsters, who won back-to-back Championship titles in 2002 and 2003, refused to give up without a fight.

She broke back in the eighth game and had two more break points in the 10th, but the determined Sharapova held firm and won through with an ace.

Although Clijsters was victorious in the first four clashes between the players, the Russian has now won their last two meetings.

"She was very accurate - especially on her serve - and kept putting me under pressure and I didn't have the chance to get into the match," said Clijsters.

"In the second set I was a little more comfortable but on important points she came up with the serves. It shows that she's the player who is hot at the moment."

Hingis got off to a flying start against Petrova as she raced into a 4-0 lead in the first set.

The Russian, who beat defending champion Mauresmo in her opener, eventually stopped the rot in the fifth game, but was unable to prevent her opponent mopping up 6-4.

It was a different story in the second as Petrova found her rhythm to level the match, but Hingis managed to raise her game in the deciding set and clinch a victory that kept her in the running for a place in the semi-finals.

Kuznetsova, who beat Dementieva in the final of the 2004 US Open, condemned her compatriot to a second straight sets defeat despite not playing at her best.

"I should have shut the match out earlier as I had a lot of chances," she said. "I lost concentration a little at times, but I'm looking forward to improving my game for the next matches."

Dementieva is now almost certain to exit the tournament after losing to the in-form Sharapova in her opening match on Tuesday.