Serena Williams became the first player to reach the semi-finals of the WTA Sony Ericsson Championship on Thursday.

Williams, who was confirmed as the year-end number one on Wednesday, beat Elena Dementieva 6-2 6-4 in Doha to complete her group campaign with a perfect 3-0 win-loss record.

The American star overcame a sloppy start which saw her drop serve in the third game to blast back in the opening set.

From 1-2 she won seven games in a row to lead by a set and a break.

Dementieva managed to stem the tide and hit back to get the second set back on serve.

But he much-talked-about serving problems were again in evidence as she produced a total of 10 double faults.

She cracked again when serving to stay alive at 4-5. She saved two match points from 0-40 but a netted forehand handed her opponent victory and with it a place in the last four.

Dementieva can still join Serena in the last four by beating Svetlana Kuznetsova in her final group match on Friday.

However, if Dementieva loses it will be Venus Williams who secures a spot in the semi-finals after she secured a 6-2 6-7 (3/7) 6-4 victory over Kuznetsova in the final match of the day in Doha.

The American had previously lost her opening two matches in the tournament against Dementieva and Serena and needed a win on Thursday to keep her campaign alive.

Venus started the match in impressive fashion as she fired down six aces to seal a 6-2 success in the opening set.

The second set, however, proved tougher for the seven-time grand slam winner as Kuznetsova produced a battling display to win the tie-break 7-3.

Both players looked in with a chance of securing victory in the decider until Venus broke at 4-4 to give herself the chance to serve for the match and put the pressure on Dementieva when she lines up in an all-Russian clash on Friday.

Earlier in the day, Caroline Wozniacki came through a dramatic contest with Vera Zvonareva - but the win may have come at a cost.

The US Open runner-up made it two wins from two matches in the White Group to close in on a semi-final spot as she won 6-0 6-7 (3/7) 6-4.

But with her injured hamstring cramping badly in the closing stages she ended the match in tears.

Wozniacki looked set for a much easier victory when she led 6-0 5-2.

Zvonareva, a late stand-in for the injured Dinara Safina who pulled out of the tournament on Wednesday, produced a string of errors as she lost the first set in less than half an hour.

She did improve at the start of the second but her chances appeared to be undermined by a nasty nosebleed she suffered when down 2-3.

She duly dropped her serve but Wozniacki could not serve out.

The Dane also missed two match points on her opponent's serve at 6-5 - the first of which was saved by a challenge to Hawk-eye after Wozniacki thought she had won the match.

Instead there was still plenty of work to do as she lost the ensuing tie-break as Zvonareva, the runner-up in last year's event, forced a deciding set.

Wozniacki secured an immediate break but then needed treatment on her hamstring when leading 3-1.

After a lengthy stoppage, Zvonareva hit back to level at 3-3 but then dropped her serve again in game nine to leave her opponent serving for the match for a second time.

Despite collapsing to the court in pain due to the cramping, Wozniacki somehow managed to seal victory.

Whether she will be fit enough to play again against Jelena Jankovic on Friday - a match which could decide whether she reaches the last four or not - remains to be seen.

As a result of her loss, Zvonareva can longer reach the semi-finals.

She also suffered an anle injury during the match and afterwards also withdrew from the event, a decision which means the second alternate, Agnieszka Radwanska, will come in to face Victoria Azarenka on Friday.