Czech Tomas Berdych stunned second seed Andy Roddick 6-7, 7-5, 7-6 on Saturday to set up a Japan Open final against Argentine Juan Martin Del Potro.
Fifth seed Del Potro, who has won 29 of his previous 30 matches, beat Frenchman Richard Gasquet 6-3, 4-6, 7-5 to book his spot in Sunday's final of the $969,000 (548,000 pound) Tokyo tournament.
Roddick, one of several players still chasing a spot in the season-ending Tennis Masters Cup in Shanghai, looked to be in control until a late collapse against ninth seed Berdych.
"The key was just trying to stay as close as possible to him," Berdych told reporters. "Even at 5-3 down it was only one break. When my chances came I just went for it."
Roddick took the first set on a tiebreak after ramming a 10th ace past Berdych.
Berdych struck back in the second, breaking Roddick's serve in the 11th game and then restored parity by forcing the American into a wild forehand.
Roddick, given a warning after slamming his racket down at the end of the second set, raced to a 3-0 lead in the decider but the former US Open champion faltered while serving for the match at 5-3 and was made to pay.
Berdych pushed the set to another tiebreak and the Czech quickly raced to a 6-1 advantage, wrapping up proceedings on his third match point with his 17th ace.
"It's frustrating," said Roddick, who won his 26th career title in Beijing last weekend. "I don't know how to rate it. You feel like you have a match in your grasp and then let it go."
Del Potro, who recently helped Argentina to reach the Davis Cup final, took just over two hours to see off fourth seed Gasquet, last year's beaten finalist in Tokyo.
The two men split the first two sets but Del Potro's nerve held in the decider, the 20-year-old securing victory and a place in the final when Gasquet blazed a forehand wide.
"The last three months have not been normal," smiled Del Potro, whose only loss in his last 30 matches was a quarter-final defeat by Briton Andy Murray at the U.S. Open.
"I just keep working because I want to be better. I don't want to relax."
A fifth title of the year would move Del Potro into eighth spot in the year-long rankings and in line for a place at next month's prestigious Masters Cup.
Rafael Nadal, Roger Federer, Novak Djokovic and Murray have already qualified for the eight-man Shanghai shootout.