Alexis Sanchez equalised for the visitors, scoring on the rebound after Manuel Neuer saved his 30th-minute penalty, but that seems of little effect now before the second leg in London on March 7.
Two more chances fell Madrid's way before the final whistle.
Real Madrid coach Zinedine Zidane believes another Champions League crown is within reach for Los Blancos.
Zidane called on his players to shut Napoli out at the Santiago Bernabeu in his pre-match conference yesterday.
Madrid will likely need Ronaldo to thrive again this time against Napoli, as Bale will not play despite having recovered from right-ankle surgery.
Mertens also saw another effort smothered by Navas from close range and Callejon a goal ruled out for offside as Napoli slumped to their first defeat in 19 games.
Real Madrid look comfortable at the top of the table and with a couple of games in hand they'll be confident of securing the La Liga title.
It wasn't even so much that they were ready for Real Madrid, it was nearly as though Napoli didn't care that they were playing Real Madrid.
Ramos is now 12th on the all-time Madrid appearances list, closing in on former teammates Roberto Carlos who is 11th on 527 and Guti who is 10th with 542, with quite a way to go to catch Raul Gonzalez who heads the list with 741 games. Madrid have scored at least one time in every match they've played this season.
Take Over 2.5 Goals and Both Teams to Score? Real Madrid have been eliminated in seven of their last eight ties with clubs from the peninsula in the European Cup or Champions League in the knock out stages; the only exception was against Roma in last season's round of 16. We know it's 50/50; we have the first game at home tomorrow and we know that we can hurt the other team and that they can cause us problems too.
Once Napoli got into a position where they needed to play catch-up in this match, Maurizio Sarri simply didn't have the tools on his bench to do it.