But Nadal was up to the task, ripping several winners on the run that were never inside the court until they landed and skipped away untouched.
Simon won their last meeting in Madrid in October.
"I was coming to the match with some doubts," Nadal said. "But I knew I was playing better here."
Simon is one of the quickest players around, but Nadal ground him down by constantly sending him sprinting from corner to corner. Even with the roof still closed, the lanky Simon was pouring out sweat.
Nadal got his seventh service break on a forehand that caught the line for a 6-5 edge in the third set, then held at love with Simon sending a forehand long on match point.
His victory set up an all-Spanish semifinal after 14th-seeded Fernando Verdasco - the lowest-ranked player to reach the quarterfinals - beat No. 5 Jo-Wilfried Tsonga, last year's runner-up, 7-6 (2), 3-6, 6-3, 6-2.
"I think it's incredible for us," Nadal said. "One will be in the finals, so we have to be happy with that."
Roger Federer, who is aiming to equal Pete Sampras' record 14 Grand Slam singles titles, plays Andy Roddick on Thursday in another semifinal.
The winners play in the final Sunday night.
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