Gregoire Defrel stunned the San Paolo when he fired Cesena in front with a powerful left-foot drive after being given too much space in the 15th minute and the home fans were growing restless until Napoli replied with two goals in a minute.
Mertens equalised in the 19th minute when Jose Callejon's shot was deflected into his path and the Belgian midfielder immediately set up a second when he weaved his way into the area to leave Manolo Gabbiadini with an easy finish.
Napoli, who have made a habit of dropping points at home to opponents from the lower half of the table, failed to capitalise and Defrel brought the Sea Horses level with his second goal on the stroke of halftime.
Napoli's fans were again growing impatient and coach Rafael Benitez began to look uncomfortable when Mertens turned the ball in from Marek Hamsik's pass just before the hour.
But the hosts still looked wobbily and were relieved to hold out for a win which kept them fourth with 63 points, three behind third-placed Lazio and four adrift of second-placed Roma.
The team which finishes second qualifies for the Champions League group stage and third place earns a spot in the final qualifying round.
Fiorentina went fifth, six points behind Napoli, and closer to a Europa League spot when they beat relegated Parma 3-0 with Gonzalo Rodriguez, Alberto Gilardino and Mohamed Salah sharing the goals.
(Writing by Brian Homewood in Berne; editing by Justin Palmer)
Share
