Mertens double grabs win for Napoli
Napoli broke the deadlock eight minutes into the second half following an entertaining first period. Gonzalo Higuain cut the ball back into the area from the right flank for Mertens to volley into the bottom right corner.
Manolo Gabbiadini hit the crossbar for Sampdoria shortly before Mertens doubled his tally with a free kick over the wall which bounced under Junior da Costa's arms.
Sampdoria had a chance to get back into the game when an effort from substitute Nicola Pozzi hit both posts but stayed out.
Napoli moved two points behind Roma, which lost to Juventus on Sunday. It was Sampdoria's first loss under Sinisa Mihajlovic.