The Spaniard trailed 5-2 in the first set to an inspired Simon, who wasted six set points, but he dug deep to win it and then ran away with the match.
"He started playing very good, very aggressive," Nadal said in a courtside interview. "I had some difficult moments, he had some set points.
"I served very well on the big points -- it is not normal that I serve two aces (on set points) but it's good." With Nadal guilty of hitting the ball too short, 28th seed Simon played superbly but once the Spaniard had pinched the first set, he was always in control.
Nadal grew in confidence as the match progressed and as Simon tired, he eased to victory, setting up a clash with either 23rd seed Paul-Henri Mathieu of France or Austrian Stefan Koubek.