Kim Clijsters sprained her ankle in the second set
Kim Clijsters faces a battle to hang on to her world number one status after she was forced to retire in the semi-finals.
Clijsters was leading third seed Anastasia Myskina 7-5 4-4 when she withdrew with a sprained ankle.
World number two Justine Henin-Hardenne thrashed Maria Vento-Kabchi of Venezuela 6-0 6-3 to reach the final.
Henin-Hardenne will move to within 187 points of Clijsters if she beats Myskina on Sunday.
Clijsters also has three titles to defend over the remaining six weeks of the season.
"Three days ago I was walking down the stairs from the players' lounge when I tripped over a step and did something to the ankle," said Clijsters.
"It didn't hurt at all initially but then last night it started to hurt for the first time.
"And today when Anastasia served wide on the first point of the eighth game (of the second set) I slid into the shot and felt a sharp pain on the inside of my right ankle.
"I tried to play after taping it but it was no good. This is really disappointing as I was really looking forward to getting to the final.
"I hope I will be okay to play in Filderstadt near Stuttgart in a week's time."
Myskina sympathised with Clijsters.
"I am still going to be in the world's top ten as a result of this but I wasn't looking to win this way," she said.
"I feel very sorry."