Andre Agassi beat Spain's Tommy Robredo 6-7, 6-3, 6-2 at the Madrid Masters to book a semi-final place against Russia's Marat Safin.
Fourth-seeded Argentine David Nalbandian earlier beat American Taylor Dent 7-6, 6-3 and will play Croatian Ivan Ljubicic, who secured a 7-6, 6-7, 7-6 victory over seventh-seeded Swede Joachim Johansson.
Safin, finally finding his form at altitude, beat Peruvian Luis Horna 6-4, 6-4 in just 80 minutes.
Although Horna, who had previously powered past defending champion Juan Carlos Ferrero, started strongly he was no match for a focused Safin.
Agassi looked set for a quick victory in his quarter-final on Friday when he broke Robredo's second service game but the tenacious Spaniard broke straight back and went on to grab a tiebreak when a nervous-looking Agassi double-faulted on set point.
Robredo made a strong start to the second set but was unable to convert any of the six break points he set up in the third game and a refocused Agassi went on to break his next serve and claim the set.
"That was a long game. And in hindsight it was the big one. he had six break points, these are the things that keep you in it." Agassi said
"I managed to break him the next game and I think that took a little out of him."
Robredo put up little resistance in the third set and Agassi easily moved through to the semi-finals.
Dent broke Nalbandian in the sixth game of the first set in their quarter-final but was broken himself while serving for the set at 5-3. The American then went 4-1 up in the tiebreak before Nalbandian won six points in a row to take the set.
There were no breaks in the first semi-final, which lasted nearly three hours with Ljubicic moving ahead in the deciding tiebreak before Johansson handed him victory with a double fault.
The Croatian finally edged ahead in the deciding tiebreak before Johansson handed him victory with a double fault.
"At the end, I served two big serves and he finished with a double fault, which was maybe the trademark of our game, that everything was decided by serves," Ljubicic said.
The unseeded Ljubicic said he was ready to take on Nalbandian after knocking out top seed Tim Henman in the last round and thought he had a chance of claiming the title.
"I do feel more and more comfortable on the court every match I play," the Croatian said. "If I beat Henman back to back with Joachim, I don't see why I cannot go all the way. I feel like I have a game for almost anybody."
