Veteran Lazio goalkeeper Pepe Reina picked the ball from the back of the net four times as leaders Napoli ran riot to extend their lead at the top of the Italian Serie A table.
Napoli were celebrating the life of the late Diego Maradona during the game and they were able to produce a first half performance that the Argentine legend would have been proud of.
The Lazio duo of Ciro Immobile and Luis Alberto tested David Ospina in the Napoli goal and he was equal to the task before central defender Francesco Acerbi struck the crossbar with all the assault on the Napoli goal coming in the space of five minutes.
Napoli upped the tempo and they were rewarded with a third goal on 29 minutes as Mertens completed his brace via a looping shot over Pepe Reina in the Lazio goal. Goals from Belgian international Mertens (brace) and Zelinksi gave Napoli a three-goal lead at the break as they produced one of their best displays in the league this season to honour Maradona.
Spanish midfielder Fabian Ruiz is renowned for scoring goals from outside the box and he achieved the feat once again when he wrapped up victory with five minutes left on the clock.
Napoli began the latest set of games in the Italian top flight with the same number of points as AC Milan in second place. They were able to move three points clear of Milan with the big win over Lazio after nearest title challengers Milan were beaten 3-1 at home by Sassuolo. Milan finished the game with a man down after skipper Alessio Romagnoli was sent off.