Mertens scores hat trick in Napoli's 5-1 rout of Empoli
Napoli extended its unbeaten run at home in all competitions to 12 matches.
Napoli moved back into second place, three points behind Juventus and three ahead of Inter Milan.
On Saturday, Juventus hosts Cagliari, and Inter plays Genoa.
Napoli hosts PSG on Tuesday.
Empoli plays a similar style to Napoli's quick-passing game and that seemed to inspire Napoli.
The last five meetings between the clubs have produced a total of 22 goals.
Nine minutes in, Insigne finished off a counterattack after being set up by Kalidou Koulibaly.
Mertens made it 2-0 before the break after Insigne intercepted a pass and set up the Belgium international for a shot from 20 meters.
Francesco Caputo pulled one back for Empoli near the hour mark but Mertens quickly responded with a spectacular dipping effort from the edge of the area.
Mertens then set up a goal for Milik, and achieved his hat trick in added time, running onto a perfectly placed pass from Insigne and evading the goalkeeper before scoring into an empty net.