Top seed Andy Roddick saved two match points before overcoming Spaniard David Ferrer 2-6, 6-3, 7-6 to reach the Paris Masters semi-finals on Friday.
The world number three, who was hampered by a sore back, won the decisive tiebreak 10-8 with a service winner after two hours 12 minutes of spectacular tennis.
The powerful American recovered from a sluggish start to set up a clash with Croat Ivan Ljubicic, who extended his brilliant indoor run by moving past Spaniard Tommy Robredo 7-6, 3-6, 6-1.
"I got out there and it was just stiff," Roddick said of his back. "It got progressively worse the more I moved. I just didn't feel real comfortable for the whole match."
Roddick said he is not sure if he would be able to play his semi-final on Saturday.
"I wish I could give you a definite answer," he said.
"If I had to play in an hour, I wouldn't be able to but a lot can be done from now until then."
GAUDIO OUT
Czech Radek Stepanek, a losing finalist here last year as a qualifier, also reached the last four, ousting third seed Nikolay Davydenko of Russia 7-5, 6-4.
Number eight seed Stepanek will next face fellow Czech Tomas Berdych, who beat seventh-seeded Argentine Gaston Gaudio 7-5, 6-0 in late action, for a place in Sunday's showpiece.
Gaudio's defeat ruined his hopes of making the Masters Cup and means Ljubicic takes the last remaining ticket for the eight-strong season finale from November 13 in Shanghai.
The Croat joins Roger Federer, Rafael Nadal, Andy Roddick, Lleyton Hewitt, Andre Agassi, Guillermo Coria and Nikolay Davydenko in the Shanghai field.
Number 10 seed Ferrer came very close to upsetting Roddick. The Spaniard served for the match, leading 5-4 in the third set, but his nerve failed him and he was broken.
The set went to a tiebreak with plenty of drama, Roddick wasting a match point before saving two, the first with an ace and the second with a superb cross-court forehand winner.
Roddick, chasing his sixth title this year, made a disastrous start, dropping serve straight away and in the fifth game before netting a backhand to lose the first set in 40 minutes.
PERFECT TIMING
Twice towards the end of that set the American received treatment on his back for several minutes.
He recovered, breaking Ferrer with perfect timing to lead 5-3 in the second and serve for the set, which he won courtesy of an unforced error from his opponent.
Ljubicic, one of the strong players of the indoor season, struggled against Robredo before stepping up a gear in the final set.
"It was an extremely difficult match, both physically and mentally," said Ljubicic.
"I am very tired but the motivation is too big to stop now," said the Croat, who has the Masters Cup and the Davis Cup final against Slovakia to look forward to.
The 26-year-old Stepanek, chasing his first title, showed signs of nerves against Davydenko when he served for the match leading 5-2 only to be broken after a double fault.
Two games later, however, he earned three match points and saw Davydenko hit a return long on the first to bow out after 72 minutes.
"I love it here," said Stepanek. "It feels like home."