Mertens almost doubled Napoli’s lead six minutes before the break when he flicked over Szczesny but it was chalked off after after the Belgian’s earlier foul on Fazio.
Roma showed promise immediately after the restart but Radja Nainggolan’s drive came off the leg of Kalidou Koulibaly.
Minutes later, Mertens doubled Napoli’s lead in style. Roma lost possession as they tried to break and after a swift exchange of passes Insigne delivered a great curling ball for Mertens at the back post where he was left unmarked by Fazio.
Roma fought back again, but a superb Reina was at full stretch to tip Diego Perotti’s curling right-footer out for a corner.
Spalletti replaced Fazio with Bruno Peres, with Mohamed Salah coming on for Stephan El Shaarawy.
Salah’s impact was immediate but the Egyptian reaped little reward for his efforts, seeing a late strike come off Reina’s post before the ‘keeper kept out Perotti’s late strike that came off the crossbar.