Second seed Nadal, who dropped only nine games in his first two rounds at Roland Garros, was never under threat from the big Argentine doubles specialist.
Schwank, ranked 192 in singles, enjoyed his time in the spotlight, milking the applause when he won a point on the Spaniard's serve midway through the final set and raising his arms to encourage the fans to cheer him on.
Nadal, seeking his 11th grand slam title, finished off with a service winner to take the match in two hours 16 minutes.
Next up is a harder task against friend and frequent practice partner
"He's one of the players I really like on the tour," Nadal told a news conference. "I really get on well with him.
"We know each other very well, personally speaking, but also from the point of tennis. We spend a lot of hours together, we practise very often together so we both know what we're going to try and do when we play together.
"We'll try and be very aggressive, we'll try and speed up and change gears, and we'll see who's going to win."
First, though, Nadal will have his 26th birthday on Sunday, though he said he had nothing special planned beyond practice and his usual treatment.
"I will do as I usually do," Nadal said. "Tomorrow is going to be a day like any other day."
Mathiieu went down 6-4 6-4 1-6 4-6 6-1, capitulating in the fifth set as fatigue took over.
The Frenchman beat big-serving American John Isner in a 76-game classic on Thursday and he showed great reserves of fight again when he fought back to level against Granollers after losing the opening two sets, whipping the partisan home crowd into a frenzy.
Spaniard Granollers regained the initiative in the decider, breaking in the fourth and sixth games to close out the match and set up a fourth-round clash with sixth seed
On a hot day at Roland Garros, Russian
"I just wanted to say sorry to the fans because they came to see a beautiful match but I simply could not give them that," Youzhny told Russian reporters after the 6-0 6-2 6-2 defeat.
The Briton will now play 17th seed
Murray said he had felt better on Friday and had done some light practice.
"I woke up this morning again feeling better than I did, but that's because of the work my physio has done the last 48 hours and all the recovery work that we've done between the last match and now," he told a news conference.
"So obviously with the rest and doing all the right things, I felt better."