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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.