Fourth seed Marat Safin stumbled to a 6-2 6-3 defeat by Robby Ginepri in the Cincinnati Masters quarter-finals.
Ginepri made only six unforced errors as Safin struggled, smashing a ball out of the stadium and throwing his racket in disgust.
He will now play top seed and world number one Roger Federer, who beat Jose Acasuso of Argentina 6-4 6-3.
Fifth seed Andy Roddick and third seed Lleyton Hewitt will meet in the other semi-final.
Roddick overcame a sluggish start and an upset stomach to beat Mikhail Youzhny 2-6 6-3 6-4 while Hewitt beat seventh seed Nikolay Davydenko 6-2 6-3 in only 67 minutes.
Federer v Ginepri
Roddick v Hewitt
Safin was struggling through the match with a knee problem but he remains hopeful he can play in the US Open.
"I didn't practice for one month and a half and I played three matches in a row and today I felt really, really tired on the court," he said.
"I couldn't move properly and I couldn't find my game. My timing was not there. So it's really difficult to play against any player."
Roddick lost his serve in the opening set against Youzhny but the Cincinnati crowd gave him a prolonged ovation at the start of the second set to lift his spirits.
"The crowd has definitely helped me through a couple of matches this week. They recognised I needed it early on in the second set," he said.
There was one break point in the final set, and Youzhny's only double-fault of the match put Roddick 3-2 ahead. A few minutes later, he got a pill from the trainer for an upset stomach, but went on to serve out for the match, hitting 14 aces.