Caroline Wozniacki of Denmark will finish at the top of the world rankings this year after she pulled off a 3-6, 6-1, 6-1 win over Francesca Schiavone at the WTA Tour Championships on Thursday.

Wozniacki, 20, became the first Danish player to achieve the feat but it was not until midway through the second set did that happen.

Wozniacki joined Australia's Samantha Stosur as a semifinals qualifier from the maroon group at the expense of Elena Dementieva, the Olympic champion from Russia, who will be out even if she beats Schiavone on Friday.

The turning point came in the fourth game of the second set when Wozniacki appealed to the Hawkeye computerised replay system.

To the surprise of many she got an "out" call on her backhand cross-court reversed -- only for Schiavone to make her own, very animated appeal.

The Italian complained that the line judge's call had denied her a chance of trying to return the ball, but the umpire disagreed.

That annoyed Schiavone, firing her up to cling on to her service game for 1-3 after a long tussle. There immediately followed another lengthy struggle in which Wozniacki was made to survive two deuces before holding serve and reaching 4-1 for a stranglehold on the second set.

In the third, the Dane saved a break back point for 2-2 and followed it by winning the rally of the match.

Kim Clijsters, playing her first tournament since the successful defence of her US Open title six weeks ago, made sure of qualifying for the semifinals with victory over Victoria Azarenka.

Clijsters overcame the world number ten from Belarus 6-4, 5-7, 6-1, guaranteeing that both she and Zvonareva, who she plays on Friday, will go through.

Earlier, Dementieva beat Stosur, the world number five, 4-6, 6-4, 7-6 (7/4), but by taking the opening set the Aussie did enough to secure her passage.