Nadal was playing his first match since withdrawing from Indian Wells with knee injury, prior to his much-anticipated semifinal match against Roger Federer.
"It has been a tough year and a half for me, so it's tough to have a clear view about how I am," Nadal said Monday. "I had too many stops; I didn't find a way to play three weeks in a row without problems. So it has been hard for me. When you have these things going on, on the mental side it's up and down. It's always like a comeback."
The 32-year-old Spaniard, who is the three-time defending champion, is bidding for his 12th Monte Carlo title. He has won 57 clay-court trophies, an Open Era record.
He plays either Grigor Dimitrov or Jan-Lennard Struff next.