Scott McTominay’s early goal moved Napoli second in Serie A as meek Cagliari lost to give their fellow strugglers the chance to make ground.
McTominay got the break of the ball off the post at a corner to force home in the second minute.
The 29-year-old threatened again later in the first half and fellow Scotland midfielder Billy Gilmour had a shot blocked inside the box before the break.
Napoli’s superiority continued in the second period with Kevin de Bruyne and Matteo Politano testing goalkeeper Elia Caprile.
Defending champions Napoli trail Inter Milan by six points. AC Milan can return to second with victory at home to Torino on Saturday. Cagliari are six points clear of the relegation zone, with the bottom five sides playing across Saturday and Sunday.