Sergio Ramos headed an equaliser and created an own goal to set Real Madrid on the way to a 3-1 win at Napoli on Tuesday which took the title-holders into the Champions League quarter-finals after they had taken a first-half battering.

The Serie A side, trailing 3-1 from the first leg of the round of 16 tie, looked more than capable of overturning the deficit as they dominated the first half and took the lead through Dries Mertens in the 24th minute.

But Napoli's hopes of a first quarter-final appearance effectively ended when defender Ramos, who has made a habit of rescuing his team, headed in from a corner in the 51st minute.

He repeated the trick six minutes later, although the ball took a deflection off Mertens and was given by UEFA as an own goal to the Belgian.

Alvaro Morata added another goal for Real in stoppage time to complete a 6-2 aggregate win which sent the Spanish side through to their seventh successive Champions League quarter-final.

Here are five things we learned.

Sergio Ramos can always be relied to score the goals that matter

Ramos scored two headers in six minutes (Getty)

Make no mistake about it, Real Madrid were in serious trouble heading out for the second-half of this match. Although they led on aggregate, Napoli had dominated the match, led 1-0 on the night and looked the team more likely to score again.

Step forward Sergio Ramos. The Madrid captain always seems to score the goals that matter — such as the late equaliser against Barcelona in El Clásico earlier this season, or his 90th minute header in the Champions League Final against Atletico Madrid 2014 — and was on hand to bail out Real's faltering front-line once again.

His two headed goals (the second later given as an own goal) at the start of the second-half killed off the tie and secured Real's progression, before Álvaro Morata fired in a rebound late on to complete an impressive comeback win.

Chelsea would be lucky to have Kalidou Koulibaly

Ronaldo struggled against Koulibaly (Getty)

It takes a special kind of defender to keep Cristiano Ronaldo quiet. And that's exactly what Koulibaly is. He repeatedly frustrated Ronaldo, dealt with the aerial threat posed by Benzema and was always tidy in possession.

The Senegal international has repeatedly been linked with a move to Chelsea in recent weeks, and the thought of him slotting into Antonio Conte's three-man defence alongside Gary Cahill and David Luiz is a tantalising prospect.

Athletic, disciplined and powerful, Koulibaly would be a superb signing for the Premier League leaders. Although playing as well as he is, Napoli will surely demand a huge fee for his services.

Real will struggle to replace Pepe

Pepe in action against Mertens (Getty)

34-year-old Pepe, who is departing the Santiago Bernabéu after ten hugely successful seasons at the end of the current campaign, was handed a difficult night at the office by Napoli's razor-sharp front-line of José Callejón, Dries Mertens and Lorenzo Insigne.

Their collective pace troubled both Pepe and Sergio Ramos, but the Portuguese centre-back still impressed with his physciality and wiliness, winning one thunderous first-half tackle to deny Mertens and generally using every trick in the book to deny the trio any goal-scoring opportunities - including theatrically throwing himself to the floor on at least one occasion.

He is nowhere near as quick as he used to be and his career at Madrid has reached its natural end, but the European champions will certainly struggle to replace him.

Mertens has been truly revitalised by Sarri

Mertens was in formidable form for Napoli (Getty)

Napoli's coach Maurizio Sarri has revolutionised the career of Belgian forward Dries Mertens. The 29-year-old was initially deployed as a left-winger at Napoli — and a back-up one at that — but has been moved into a central role by Sarri where he has flourished. He is well in contention to win the Serie A's Capocannoniere top goal-scorer award, just one goal behind Gonzalo Higuain, and was Napoli's best player against Real.

He opened the scoring with a fine low shot past Keylor Navas after being played in by Marek Hamšík and repeatedly exploited the space between Real's midfield and defence, drifting between the lines with confidence.

For a player fast approaching 30, his transformation is nothing short of remarkable and he was both unlucky to be denied two second-half penalties, and to find himself on the losing team.

Napoli supporters taking nothing for granted

Napoli supporters arrived at their stadium five hours early (Getty)

What a contrast the Stadio San Paolo made from the Emirates Stadium in the hours ahead of kick-off. While Arsenal fans largely eschewed supporting their team in favour of protesting against Arsene Wenger, Napoli supporters arrived at their stadium up to five hours early to begin generating an intimidating atmosphere as their team prepared to overturn a 3-1 deficit.

Some die-hard supporters even began camping outside Madrid's team hotel on Monday, singing in an attempt to keep the team awake throughout the night.