Napoli made it two wins from two in Serie A to keep pace with leaders Juventus but their victory owed a lot to the poor finishing of Fiorentina striker Stevan Jovetic.
The Montenegro international scored a late goal, which turned out to be nothing more than a consolation for Fiorentina, after missing several earlier opportunities.
Napoli were already two goals to the good, with a Marek Hamsik header going in off Borja Valero and Blerim Dzemaili also netting, before Jovetic finally found the target.
Fiorentina, who suffered their first defeat of the league season, dominated the early proceedings but could not break the deadlock.
Jovetic beat Hugo Campagnaro after three minutes but shot wide and wasted another chance following a corner as he fired at Morgan De Sanctis.
The Fiorentina goalkeeper then watched Juan Cuadrado shoot wide before stopping a 30-yard effort from the same player.
Five minutes into the second half Cuadrado tried his luck with another long distance shot but his effort was again saved.
Five minutes later Fiorentina were made to pay for their missed chances as Camilo Zuniga's free-kick was headed towards goal by Hamsik with the ball hitting Valero before crossing the line for the opening goal.
Jovetic missed another chance after 65 minutes when his ambitious effort from long range went high over the bar.
With 15 minutes to go Napoli increased their lead as Dzemaili scored from the edge of the area with a shot which looked to wrap up the three points for the home side.
With just three minutes remaining Fiorentina finally got the goal their play deserved when Jovetic volleyed in.
But Napoli were able to see time out to claim a victory which keeps their winning start to the season intact.