World number one Serena Williams marched into the semi-finals of the WTA Championships on Thursday with a comfortable win over Elena Dementieva while sister Venus kept her title hopes alive long after midnight.

Serena's 6-2 6-4 victory completed a 100 percent record for the American in Maroon Group and confirmed her as the first player through to the last four at the season-ending showpiece.

Venus, who suffered narrow defeats to Dementieva and Serena in her first two matches, beat Svetlana Kuznetsova 6-2 6-7 6-4 in a hard-hitting clash and must now sweat on Friday's results before knowing whether she will be in action at the weekend.

Feisty teenager Caroline Wozniacki remained undefeated on her debut at the $4.5 million Championships after beating Russian stand-in Vera Zvonareva in a White Group match that at times resembled a hospital casualty department.

The 19-year-old Dane eventually won 6-0 6-7 6-4 but played the last four points virtually on one leg. She was in tears after suffering agonising cramps in her heavily-bandaged left thigh and could barely hobble to the net to shake hands.

Zvonareva, a replacement for fellow Russian Dinara Safina who withdrew with a back injury, was also in the wars, needing treatment in the second set as blood oozed from her nose.

The Russian later withdrew with an ankle injury meaning second reserve Agnieszka Radwanska of Poland will get a run out against semi-final hopeful Azarenka on Friday.

"I have absolutely no idea how I pulled it through, but I'm very happy about it," said surprise U.S. Open runner-up Wozniacki, who may still need to beat Serbia's Jelena Jankovic on Friday to clinch a semi-final spot."

"I'm gonna do everything I can to get ready for tomorrow," added the world number four. "I'm just going to do everything that physio and the doctors are saying, drinking a lot of fluids, eat some good food, stretch, get some massage, get some ice massage, take a salt bath, everything."

There were no such dramas for Serena who recovered from an early service break to win seven games in a row against Dementieva, a player she had lost to three times this year.

Danish teen Caroline Wozniacki, never before in the last eight at a Grand Slam, advanced to the US Open final by defeating Belgium's 50th-ranked Yanina Wickmayer 6-3, 6-3, on Saturday.

Wozniacki, the ninth seed, won a matchup of 19-year-olds to put herself into Sunday's championship match against Belgium's Kim Clijsters, who dethroned defending champion Serena Williams of the United States 6-4, 7-5.

"It feels fantastic," Wozniacki said. "I'm really excited."

Wozniacki, who has never played Clijsters, won her 11th match in a row, including a title run at New Haven the week before the US Open began, and is set to reach a career-best seventh in the next world rankings.

Nearly eight hours of drizzle delayed the start of the match and forced it to be moved from Arthur Ashe Stadium to the secondary Louis Armstrong Stadium once the showers stopped and the courts could be dried.

After losing the opening point on her serve, Wickmayer complained to umpire Lynn Welch about the slick surface, sliding her foot along the court to a near-splits position to show her concerns.

Welch sent both players to their chairs, scuffed the lines with her feet and called for the lines to be toweled off. After another test, she said, 'Seems OK now' and sent the players back onto the court.

Wozniacki and Wickmayer exchanged service breaks to open the match but the Dane broke again in the fifth and seventh games for a 5-2 edge, the latter when Wickmayer netted a forehand after a lengthy rally. Wickmayer broke back at love to 5-3 but surrendered four of the next five points, netting a backhand volley to give up the break and drop the first set after 49 minutes.

Wickmayer broke for a 3-2 edge in the second set, winning a review appeal on a forehand volley to seize the lead, but sent a backhand wide to give back the break in the next game.Wozniacki took the critical break in the eighth game when Wickmayer netted a backhand, then held serve to win the match. Wozniacki sent a backhand long on the first match point but Wickmayer was wide with a crosscourt forehand volley on the second to hand Wozniacki the victory after 96 minutes.

Wickmayer, whose mother died of cancer at age nine and whose father gave up his construction job so his daughter could follow her tennis dream, had longed for a finals matchup against her idol Clijsters.  Instead, Wozniacki became the first Danish man or woman to reach a Grand Slam final, surpassing the 1978 Australian Open women's quarter-finals run by Dorte Ekner as her homeland's best showing.

Completing the semi-finals Saturday averted the first US Open women's singles final on a Monday since 1974, provided Sunday's sunny forecast holds.

Caroline Wozniacki of Denmark has upset two-time major champion Svetlana Kuznetsova of Russia at the U.S. Open to reach her first Grand Slam quarterfinal.

The No. 9-seeded Wozniacki came back to beat No. 6 Kuznetsova 2-6, 7-6 (5), 7-6 (3) Monday night.

Wozniacki will face 17-year-old American Melanie Oudin next. All four of the women left on their half of the draw will be playing in a major quarterfinal for the first time.

Kuznetsova won the 2004 U.S. Open and this year's French Open.