Italy - Lorenzo Insigne beat former Italy team-mate Gianluigi Buffon from the spot to keep Napoli well in the hunt for a place in the last 16 of the Champions League thanks to a 1-1 draw at home to Paris Saint-Germain on Tuesday.
PSG travelled to the Stadio San Paolo looking to go one better after being held to a 2-2 draw in the French capital two weeks ago.
But after Juan Bernat capped a dominant first-half performance from PSG to hit the opener on the stroke of half-time, Insigne made no mistake when the hosts were awarded a penalty just after the hour after Jose Callejon was sandwiched in the area.
The result sees Napoli go top of Group C, level on points with Liverpool following their shock 2-0 defeat by Red Star Belgrade earlier Tuesday. PSG sit third, only one point adrift.
In what was captain Marek Hamsik's record 512th appearance for Napoli, former team-mate Edinson Cavani played only a cameo role after replacing Angel Di Maria for the final quarter-hour.
It left Neymar and Kylian Mbappe pulling the strings up front for the visitors, who produced a strong first 45 minutes that had Napoli's demanding fans frustrated throughout.
When Neymar waltzed through the Napoli defence on 18 minutes it took a timely interception from Mario Rui to avert the danger.
But the first real scare for Napoli came on 23 minutes when Mbappe ran on to Neymar's looping cross to fire a left-footed half volley that dipped just over David Ospina's crossbar.
After Mertens poked wide of Buffon's upright after a well-worked free-kick, Kalidou Koulibaly rescued the Italians twice in succession with a well-timed tackle on Mbappe and a timely interception to prevent Mbappe reaching Thomas Meunier's pass.
At the other end Callejon set up Insigne for a deft volley which the diminutive Napoli midfielder fired a yard wide of Buffon's upright.
But Napoli's fans were in despair moments later courtesy of more Mbappe magic.
The World Cup star's pace carried him on to a cross down the left and he bamboozled Raul Albiol before sending a deft cutback into the path of Bernat.
The Spaniard took a touch and, as he fell to the turf, swiped with his right foot to stun Ospina.
A half-time pep talk by coach Carlo Ancelotti appeared to have had the desired effect when a far more enterprising Napoli emerged after the interval, 40-year-old Buffon rolling back the years with some fine saves.
The former World Cup winner first stretched to palm Nikola Maksimovic's angled effort to safety before guiding Mertens' lobbed effort over the bar moments later.
Napoli should have scored when acrobatic control by Mertens set up Callejon to the right of Buffon's goal, but the Spaniard bundled wide.
As PSG panicked at the back, Buffon palmed Callejon's close-range cross into the path of Mertens only for the Belgian to fire over from two yards out.
Napoli fans were further frustrated when former Juventus legend Buffon stood his ground to parry Fabian Ruiz's first-timer following Mertens' cross.
Buffon was finally beaten, but only from the penalty spot, by Insigne's 62nd-minute spot-kick.
PSG coach Thomas Tuchel introduced Cavani for the final 14 minutes while Mertens, holding an injured arm, was replaced by Adam Ounas.
The much-needed boost in attack failed to materialise for PSG, who five minutes from time saw Mbappe somehow sweep his strike from Thilo Kehrer's delivery wide of Ospina's upright when eight yards from goal.