WTA Championships, Doha
Dates: 26-31 October
Coverage: Live coverage on Eurosport (1430 BST) and
reports and photos on BBC Sport website
Wozniacki had to dig deep against the French Open champion
Caroline Wozniacki secured the year-end number one ranking by beating Francesca Schiavone in Doha to qualify for the semi-finals of the WTA Championships.
The Dane battled to a 3-6 6-1 6-1 win in her final round-robin match and so ensured she will stay ahead of Vera Zvonareva in the rankings this year.
Sam Stosur also made the Doha last four despite defeat by Elena Dementieva.
Kim Clijsters beat Victoria Azarenka in the day's final match to complete the semis line-up alongside Zvonareva.
Clijsters was not at her very best but did enough to defeat Belarussian Azarenka 6-4 5-7 6-1 and she will meet Zvonareva in their final round-robin match on Friday, with the winner topping the White Group and going on to face Stosur in the last four.
If you win a Grand Slam, people will say it was a lucky shot or an easy draw. For me, the most important thing is that I know I had a great season
"It was a good match and obviously disappointing to lose that second set," said Clijsters.
"It was close. She dropped her level a little bit in third set and everything she was doing well in second set she kind of lost that a little. I was happy with that. I just tried to be aggressive, focused and not let her get back in the match."
The 20-year-old Wozniacki had succumbed in straight sets to Stosur on Wednesday after an opening win over Dementieva, and she looked to be on her way out when Schiavone found her best form to take the opening set in Thursday's second match.
However, the Dane showed her mettle by forcing her way back into the match and her unerring accuracy proved too much for French Open champion Schiavone.
Wozniacki has already won six titles this season and claimed the number one spot earlier this month after reaching the quarter-finals of the China Open, which she went on to win.
She becomes the 10th player to end the season as the world number one since the WTA ranking system was introduced in 1975, and only Martina Hingis, Monica Seles and Steffi Graf did so at a younger age.
"It's something I've dreamed about since I was a little girl and I'm really happy and really proud about what I've achieved this year," she said.
And to those who doubt her status as she has yet to win a Grand Slam, Wozniacki added: "To be honest, there will be always sceptics. There are always people saying you'll never reach the top 10, never reach the top five and you'll never win a big tournament.
"If you win a Grand Slam, people will say it was a lucky shot or an easy draw. For me, the most important thing is that I know I had a great season."
Wozniacki's win also secured her a semi-final spot in Doha because Dementieva cannot overtake her in the Maroon Group standings even with victory over Schiavone on Friday.
The Russian can only match Wozniacki's 4-3 set record and even with a 6-0 6-0 victory would not overhaul the Dane's games won percentage.
Stosur was earlier beaten 4-6 6-4 7-6 (7-4) by Dementieva but had done enough by taking the first set to progress from the Maroon Group.
The Australian had lost her opening four games of the tournament but looked sharp from then on, recovering to beat Schiavone in straight sets before defeating top seed Wozniacki on Wednesday.
And the Australian looked on course for a third successive win before Dementieva worked her way back into Thursday's opening match following a poor start.
Three double-faults played a major part in Dementieva dropping serve in game four but she reeled off four games in a row for 4-4, only to see Stosur break once again and close out the set.
But having secured the set that guaranteed a semi-final place, Stosur then struggled to contain a more positive Dementieva.
The Russian needed two breaks before taking the second set and then edged the decider in a tie-break as Stosur's aggressive game faded.
"I would have preferred to have won, there's not a doubt," said Stosur. "To lose 7-6 in the third, it's not fun, but at least you know it's a matter of a few points here and there. That's tennis. I'm not going to dwell on it."
Friday's order of play (from 1500 BST):
1. Francesca Schiavone v Elena Dementieva
2. Vera Zvonareva v Kim Clijsters
3. Jelena Jankovic v Victoria Azarenka