Eleven-time Monte Carlo champion Nadal had no problems in his match as he dismissed Grigor Dimitrov 6-1 6-1 in 55 minutes.
Novak Djokovic admitted he played an "awful" match against Dan Evans at the Monte Carlo Masters as Rafael Nadal advanced with ease to the quarter-finals.
World number one Djokovic, playing in his first tournament since winning a ninth Australian Open in February, suffered his first defeat of 2021 on Thursday, DPA reported.
Evans, who had never before reached the last eight of a Masters 1000 tournament, won 6-4 7-5 with a performance that belied his relative lack of success on clay.
The 30-year-old Briton will now meet David Goffin, who surprised fifth seed Alexander Zverev 6-4 7-6 (9-7).
"To be honest, this has been probably one of the worst matches and performances from my side I can recall in the last years," said Djokovic. "I don't want to take anything away from his win, but from my side, I just felt awful on the court overall. Just nothing worked. It's one of those days."
Saying he felt "completely the opposite" to Wednesday's win over Jannik Sinner, Djokovic added: "Just was obviously very, very windy, tough to play in these kinds of conditions against a guy like Evans who makes you move. He's very unpredictable with his shots. He dismantled my game."
Eleven-time Monte Carlo champion Nadal had no such problems in his match as he dismissed Grigor Dimitrov 6-1 6-1 in 55 minutes.
Nadal, who boasts a 14-1 career record against the Bulgarian, did not face a break point in either set against his off-colour opponent.
Dimitrov started the match with three double faults in his opening service game and was 4-0 down in each set.
"(I am) sorry for him. He played a bad match. That is the truth," said Spaniard Nadal. "He made a lot of mistakes. I was there. I was doing the right thing, but it is true that today was more his fault than my good tennis."
Nadal will face Andrey Rublev after the Russian battled past Roberto Bautista-Agut 7-6 (7-2) 5-7 6-3.
Stefanos Tsitsipas, who beat Cristian Garin 6-3 6-4, will face Alejandro Davidovich Fokina next, while Fabio Fognini's straight-sets win over Filip Krajinovic set up a meeting with Casper Ruud.
