His second header goal shut the door on Napoli's chances, with Real cruising the rest of the way.
News from the Madrid camp, however, is hardly encouraging (from the Napoli viewpoint) with Welshman Gareth Bale, Frenchman Karim Benzema and footballer of the year, Cristiano Ronaldo, all ready for duty.
While not in a hole as deep as Arsenal's, Napoli are certainly in trouble heading into the second leg of this tie.
Zidane's men are firmly in control and if Napoli are to go through on away goals, the Serie A side will have to do what no team has managed since Wolfsburg beat Real in a quarter-final first leg almost a year and 46 games ago - keep a clean sheet against the reigning Champions League winners.
Mertens then hit the post after Alan's pass was deflected into his path, but at halftime it was 1-0 with Napoli earning its lead, having had 11 shots, more than any other side had managed against Madrid since Zinedine Zidane took over.
Real Madrid nearly levelled on 29 minutes when Ronaldo burst into the area and saw his effort beat Jose Reina but rebound off the base of the post.
"Champions League games are always hard". I'm not anxious, I know that we are playing against a great team. As the media is all in the North and slaves of the Northern clubs, and they all hate me, they just wanted to stir up trouble and not reflect what I really said.
"Obviously they will play with a lot of intensity at the start of the game". Yet, in a 2-1 away defeat of second placed Roma last Saturday, Napoli looked, as they have often looked in the last two seasons, like the second best side in contemporary Italian football, second only to Juventus. But a heated stadium like this is a tremendous motivation for us.
Both the teams come into this match with great confidence.
Daycare worker accused of pushing 4-year-old girl down stairs
- Police in Upper Darby held a press conference Monday that addressed the disturbing moment a day care worker pushed a child. There's no information regarding whether the camera's installation was the result of ongoing complaints about Sarah Gable .
That proved pretty much conclusive as Napoli struggled to regroup and reassert themselves in the match.
Super Stat: The last time Madrid travelled to Napoli was in 1987, when the UEFA Champions League was called the European Cup and the tie ended in a 1-1 draw!
Ramos' outburst led to claims in the Spanish press of a rift in the Real camp and a heated dressing room discussion.
Bayern Munich were too good for the Gunners in London and just as they did two weeks ago, the Germans enjoyed an emphatic 5-1 victory. However, our fans are world champions as well - and all of Europe will see that. Or do you think Real Madrid has this one in the bag?
"For me, the pressure's on them".
Kick-off is set for 7.45pm GMT.
Follow our LIVE COVERAGE here!
India: TV: Ten 1.