Second seed Rafael Nadal fell to a 6-1 6-2 defeat to David Nalbandian in the Madrid Masters quarter-finals.
The world number two lost five service games as his unseeded Argentine opponent took his chances to set up a semi-final against Novak Djokovic.
Nadal, who completed a gruelling victory over Andy Murray on Thursday, looked out of sorts throughout.
In Friday's earlier quarter-final, Serbia's Djokovic secured a 7-6 (7-2) 6-2 win over Mario Ancic of Croatia.
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And top seed Roger Federer beat Spain's Feliciano Lopez to claim his place in the last four.
It was a tight affair though and Federer had to work hard for his 7-6 (7-4) 6-4 victory.
Lopez weathered five break points in the eighth game of the first set and also saved a set point, but was unable to take his chance in the tie-break.
Federer then broke serve in the ninth game of the second set to set up victory.
Federer will play unseeded Nicolas Kiefer in the last four after the German beat fifth seed Fernando Gonzalez 7-6 (7-5) 6-2.
Nadal struggled with his ground strokes and was unable to find the form that he showed against Andy Murray 24 hours earlier.
"It is a game that isn't easy to analyse," said Nadal. "David played a great match and for me it was difficult to play worse.
"Everything went wrong for me out there. I had a clear idea of how I wanted to play but it wasn't my day."
The win put Nalbandian, a runner up in Madrid in 2004, into his first semi-final of the year after a disappointing season.
In the first quarter-final, Djokovic became the first player to take a set off unseeded Croatian Ancic in the tournament when he claimed the hard-fought opener on a tie-break.
The world number three then twice broke his opponent's serve in the second set for an ultimately comfortable victory.
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