DOHA (Reuters) - 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 teen-ager 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 agonizing 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 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 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.
SERENA FATIGUE
Dementieva's cause was not helped by 10 double faults but the Olympic champion can still reach the semi-finals here for the second year in succession by beating Kuznetsova Friday.
Serena's three wins in three days means the others are scrapping for second spot. Venus's late victory means that should Kuznetsova beat Dementieva they would all have one victory and calculators would be needed to see who survives.
"It feels good to be through because I've never really done well in this round-robin format," Serena, who won the tournament in 2001 when it was a conventional knockout draw, said at courtside. "Tomorrow I'm going to sleep all day because I've played every day so far here and I'm really tired.
Wozniacki, who needed a minute under three hours to subdue Azarenka Wednesday, looked set for a quick victory when she raced through the opening set against Zvonareva.