The second seed had endured a few tricky moments early on against Lacko but he raised his game to secure a 6-2 6-4 6-2 win in just under two hours.
And Nadal was pleased with the way he responded against the Slovakian qualifier.
"The match was a really complete match, a really solid one," he said. "I'm very happy about my game.
"Being in the fourth round without losing a set, it's fantastic news.
"I am playing well. If you are playing well, things can be a little bit less difficult. If you are playing bad, every match will be very, very difficult.
"I played three matches at a very good level. Today [Friday] was one of the best, solid, without mistakes, comfortable with the serve, having a good return.
"So nothing is easy. Every match you have your problems."
Lacko dropped his serve in the opening game but then hit back with some powerful shots to level the scores.
However, once Nadal broke Lacko's serve twice in succession to lead 5-2, the tone for the rest of the match had been set.
An early break in the second set maintained Nadal's momentum and two more in the third wrapped up what ultimately was a routine win.