"Right now I am mentally and physically tired," said Clijsters after being beaten 1-6, 6-3, 6-4. "I do not think I recovered very well after playing a late doubles match last night and this was one match too many."
Clijsters's loss -- only her 12th in 93 matches this year -- gave her compatriot Justine Henin-Hardenne the chance to steal the number one spot if she can beat Yugoslav Dokic in Sunday's final.
"I will be happy for her if she does it," Clijsters said. "I have always said that if it had to change then it should go to another Belgian.
"She has had a great year and she is probably the most improved player on the Tour this season."
Clijsters, who regularly plays doubles, is the first woman since Martina Navratilova in 1986 to play more than 90 matches in a year.
She dropped out one round earlier last year in Zurich when she lost to eventual runner-up Lindsay Davenport in the quarter-finals after being distracted by playing the dice game Yahtzee into the night with her boyfriend, men's world number eight Lleyton Hewitt.
This year Hewitt was not around and the Belgian progressed through the tournament with a bye in the first round and victories over Slovenians Katarina Srebotnik and Tina Pisnik for the loss of just nine games.
Clijsters's tennis has been energetic all week and before her semi-final she joked: "No Yahtzee, that's why. Last year it was a lot of fun but sometimes until 12 o'clock at night we just keep playing Yahtzee."
