Justine Henin-Hardenne beat Maria Sharapova to reach the final of the WTA Championships in Madrid and guarantee the year-end world number one ranking.
Henin-Hardenne reached all four Grand Slam finals in 2006
The Belgian dominated from the outset, breaking serve immediately and battling through to a 6-2 7-6 (7-5) win.
Amelie Mauresmo beat Kim Clijsters 6-2 3-6 6-3 in the second semi-final.
Sharapova went into the match on a 19-match winning streak and knowing she had to win the title to take the number one ranking from Mauresmo.
Henin-Hardenne had to beat Sharapova and reach the final to take top spot.
I'm number one but I still want to win this tournament
Sharapova was the only player to win all three round-robin matches and was the favourite to beat Henin-Hardenne, who is playing her first event since the US Open.
But the Russian looked out of sorts from the start of their semi-final, failing to find any rhythm on her groundstrokes.
Henin-Hardenne grabbed two breaks of serve and, although Sharapova got one back, she could not save the set.
The match looked all but over when Henin-Hardenne converted her fifth break point in a dramatic eighth game of the second set, but she failed to serve out the match.
Hopefully Justine will party tonight with her number one ranking
It came down to a tie-break and successive Sharapova errors gave Henin-Hardenne two match points, the second of which she converted.
"It's amazing, I'm very happy," said Henin-Hardenne. "I'm very happy with the way I handled the pressure today.
"The season is not over yet. I'm number one but I still want to win this tournament."
Sharapova said: "I just didn't feel like myself today. I felt a little sluggish. I didn't feel like I was concentrating.
"You hope to find a way to break through that, and I felt like I was getting there at the end of the match, but I just couldn't put two or three good points together."
In the second semi-final, Mauresmo broke Clijsters in the eighth game of the third set when the Belgian challenged a baseline call that went against her.
Mauresmo served out for the win to set up a rematch with Henin-Hardenne, who she beat on Friday to advance to the last four.
606 DEBATE: Who do you think will take the title?
"It's the last match of the year and obviously it's great to have Justine and I in a final again," said Mauresmo.
"Hopefully she'll party tonight with her number one ranking."