LOS ANGELES -- Jennifer Capriati fought off four set points and defeated Ai Sugiyama 7-5, 7-6 (3) in round-robin play on opening night of the WTA Championships on Wednesday.

World No. 1 and defending champion Kim Clijsters defeated Elena Dementieva 6-2, 6-2 in a match that ended just before midnight Pacific time.

"I'm sure I'll be over my jet lag now,'' said Clijsters, who had been going to sleep about 9 p.m. and waking up at 5 a.m. since arriving from Belgium last weekend.

Clijsters will play Chanda Rubin on Thursday, when No. 2 Justine Henin-Hardenne opens play against Anastasia Myskina. Dementieva will play Amelie Mauresmo, who lost to Rubin 4-6, 6-4, 6-2.

Clijsters was aggressive from the start against Dementieva, the 22-year-old Russian who lost in the first round of the championships the previous two years when it was a single-elimination format.

"I was really consistent,'' Clijsters said. "I didn't have a moment where I was down, where I was giving her easy mistakes.''

Capriati left the court leading 2-1 in the second set to have her right thigh taped after straining her hip flexor.

"This morning in practice I sort of felt something already, but I haven't been playing indoors and just not playing a lot of matches,'' she said, adding that her right hip is a weak point that often acts up.

"I served really well when I had to and it definitely saved me in the match,'' said Capriati, who lost to Serena Williams in the semifinals here last year.

Capriati returned from the medical break to take a 3-1 lead before Sugiyama won three straight games and led 4-3.

Capriati overcame a break point against her and evened the set 4-4. Sugiyama held for a 5-4 lead and had her first set point in the 10th game before Capriati won three straight points for a 5-5 tie.

Sugiyama held for a 6-5 lead, then had triple set point on Capriati's serve. Buoyed by the partisan crowd, Capriati won 10 straight points -- the final five of the 12th game and the first five of the tiebreaker. She closed out the match with a forehand winner.

Capriati was distracted by overhead strobe lights that malfunctioned in the first set.

"It was going off in between the points a lot,'' she said. "I just have to get those things out of my mind and try to focus and not let them bother me.''

Rubin rallied from a set and a service break down to beat Mauresmo in the night's only three-set match.

It was Rubin's first victory in her fourth appearance in the season-ending championships, which will determine the No. 1 ranking for the year. Rubin, ranked No. 10, got into the eight-player tournament when Venus Williams withdrew because of injury.

Rubin needed to reach last week's semifinals in Philadelphia to qualify on her own, but she lost in the quarterfinals. After Williams pulled out, Rubin found out Sunday she was headed to Los Angeles.

"She was mad at me,'' Rubin said about Williams' reaction. "She said I should have won in Philadelphia.''

Rubin broke Mauresmo with a forehand winner to close out the 2-hour, 8-minute match in front of 5,281 people at 20,000-seat Staples Center.

"I tried to force the play and be aggressive,'' Rubin said. "After being down a set and a break, I felt like I had new life.''

Mauresmo, ranked seventh in the world, earned the only break of the first set. Tied 3-3 in the second set, Rubin held after nine deuces and three break points for a 4-3 lead. Mauresmo broke after seven deuces to trail 5-4, then was broken to lose the second set.

Rubin got an early break and raced to a 4-1 lead in the third.

"She was playing better and better as the match went on,'' Mauresmo said. "She puts a lot of speed on the ball and she can also come in. Sometimes I was too far from the baseline to take control of the points.''

Rubin had more unforced errors (41) than winners (37) in beating Mauresmo for the third time in four career meetings.

The two players with the best records in the Red and Black groups will advance to Sunday's semifinals.

Venus Williams, her sister Serena, and Lindsay Davenport all missed the tournament because of injuries.