Vandevoordt, aged 17 years and 287 days, became the youngest goalkeeper in Champions League history but the teenager was badly at fault for the opening goal at the San Paolo.

Milik added a second before the half-hour and then completed his hat-trick on 38 minutes from the spot after the hapless Vandevoordt brought down Jose Callejon.

Dries Mertens got a fourth with another penalty as Napoli sealed second place in Group E behind defending champions Liverpool, who won 2-0 away to Salzburg in Austria.

Carlo Ancelotti's position as Napoli boss had come under increasing scrutiny amid a nine-match winless dating back to an October victory at Salzburg.

But this win will help alleviate some of the pressure on a coach who has won three Champions League titles, twice with AC Milan and once with Real Madrid.

Last season's Serie A runners-up needed a point to clinch a place in the knockout rounds, and they were ahead inside three minutes after a glaring error by Vandevoordt.

Moments after Kalidou Koulibaly headed against the crossbar, Vandevoordt attempted an elaborate turn inside his six-yard box but was soon closed down with Milik stabbing home.

Paul Onuachu spurned a chance to level for Genk before Milik made it 2-0 on 26 minutes with a clinical first-time finish after a curling low cross by Giovanni Di Lorenzo.

Vandevoordt's night went from bad to worse when he fouled Callejon as the Spaniard burst through on goal, with Milik converting the spot-kick for Napoli's first Champions League hat-trick.

Napoli goalkeeper Alex Meret then tipped over superbly from Onuachu before the break.

The hosts were awarded a second penalty on 75 minutes when Casper De Norre handled a Callejon volley. Mertens dispatched from the spot to draw to within three of Marek Hamsik's club record 121 goals.

Theo Bongonda hit the post late on for Genk as the Belgians champions bowed out bottom of the group with just a single point.

mw/td