Wimbledon champion Venus Williams suffered a shock 4-6, 7-6, 6-4 defeat by France's Virginie Razzano in the Japan Open final on Saturday, while top seed David Ferrer and Frenchman Richard Gasquet advanced to the men's final.
Williams, the women's top seed and winner of last week's Korea Open, blew three match points in the second set tiebreak before Razzano levelled the Tokyo final at one set apiece.
Williams, who was chasing a 37th career title, called an injury timeout for treatment to her right thigh after the first game of the decider but it made no difference.
Razzano's tenacity earned her the decisive break at 5-4 in the third, and the fifth seed sealed a stunning upset by forcing Williams into another wild forehand after two hours, 48 minutes.
"My leg started bothering me in the first set and then you know it's going to hurt the whole match," the former world number one told reporters.
"I decided to play because I wanted to win and I have no regrets. But in the third set when I got up to serve I couldn't get my leg up."
Williams praised Razzano's resilience in saving three match points.
"She played good at all the right moments," said the American, who had won 19 of 21 matches since winning her sixth grand slam singles crown at Wimbledon.
"Congratulations to her. It's definitely a great way for her to end the year."
BREAKTHROUGH WIN
Razzano's victory gave her a second title on the WTA Tour, coming just a week after her breakthrough win in Guangzhou last week.
"I never felt the match was lost, even with three match points against me," said the 24-year-old, whose run in Tokyo will see her climb into the top 30 for the first time.
"I played my best tennis at those moments. I tried to play simple and stay aggressive. It was a good system for me."
Both Ferrer and Gasquet came through their semi-finals with carbon copy 7-6, 6-3 victories.
Ferrer, who reached his first Grand Slam semi-final at this year's US Open, defused Croatian Ivo Karlovic's fierce serve in an absorbing contest.
"It's difficult when 'Karlo' is serving because he has the best serve in the game," said the Spanish world number eight.
"I maybe had some luck at the important moments."
Third seed Gasquet earned the right to face Ferrer in Sunday's final of the $832,000 men's tournament after beating number two seed Tomas Berdych of the Czech Republic.