Napoli's victory over Sampdoria has cut the gap on second-placed AS Roma to two points in Serie A.
Belgian winger Mertens provided the breakthrough for the home team after 53 minutes when he volleyed in Gonzalo Higuain's cross, just as Sampdoria were threatening to take control.
Manolo Gabbiadini blasted against the crossbar as the visitors almost grabbed a quick equaliser, only for Mertens to make the game safe by curling in a 25-metre free kick in the 62nd minute.
Roma suffer 3-0 loss
The win gave third-placed Napoli 39 points from 18 games, two behind Roma who lost 3-0 at leaders Juventus on Sunday.
In their previous six games, Rafael Benitez's Napoli had lost 3-0 to Juve and 1-0 at home to Parma, beaten Lazio and
Inter Milan and dropped points at home to Udinese in a 3-3 draw and in a 1-1 draw at Cagliari.
It was sixth from bottom Sampdoria's first defeat in six games since Sinisa Mihajlovic took over as coach.