Piatek - signed by the Rossoneri from Genoa to replace the departed Gonzalo Higuain - quickly endeared himself to Milan fans, striking twice in the first half to become an instant hero with the San Siro faithful.

The Poland international had made his debut off the bench in the 0-0 draw between the teams last Saturday, yet his finishing abilities made sure there was no repeat of that Serie A stalemate.

Both times he capitalised on space available through Napoli's right channel; his opener was tucked home with finesse when clean through but his second was all about power, a right-footed rocket fizzing past visiting goalkeeper Alex Meret.

The striker had already slashed an ambitious first-time volley wide before he opened his Milan account 10 minutes into the tie, Diego Laxalt's hopeful long punt putting his team-mate in the clear.

Piatek – who had scored four goals for Genoa in his maiden Coppa Italia appearance, against Lecce in August – kept his cool to convert, opening his body up to bend a right-footed effort beyond Meret.

He doubled both his tally and Milan's lead 17 minutes later, only this time his attempt with the same foot came when he was going away from goal, Piatek cleverly keeping the ball away from Kalidou Koulibaly as he cut inside from Milan's left before unleashing a drive that flashed into the net.

Napoli, in contrast, struggled to create opportunities despite dominating possession. Lorenzo Insigne missed an excellent chance to equalise for the visitors soon after Piatek had broken the deadlock, carelessly sending a first-time effort straight at Gianluigi Donnarumma 

Alessio Romagnoli's block denied Arkadiusz Milik tapping home a loose ball following a corner after the break, with Milan holding firm to keep a clean sheet and become the first team through to the last four.