It was a shaky start between two teams who showed promise in the first half of the season, but the added motivation of being able to top the Serie A meant that much of the positive play went Napoli's way.
The Partenopei were unlucky to have Mathias Olivera offside when he scored after 15 minutes, before Fiorentina's David de Gea was in top form to deny Leonardo Spinazzola's effort.
The visitors kept pushing and eventually the pressure became too much for the Viola.
David Neres brilliantly managed to deny Fabiano Parisi and enter the penalty area before unleashing a powerful strike into the roof of the net at De Gea's near post.
Napoli were undefeated this season when they scored first, but that didn't stop Fiorentina from hitting back straight away. A dominant spell culminated in the Viola not allowing itself to be cast afterwards
Moise Kean scored, which particularly infuriated manager Raffaele Palladino, as the light touch of Kean's hand on the ball was likely caused by contact from Giovanni Di Lorenzo.
More efforts came from Rolando Mandragora and Dodo, but Fiorentina fell behind at half-time for the first time in Serie A since facing Lazio here in September – a match they eventually won.
However, a repeat of this looked unlikely after debutant Matías Moreno brought down Frank Anguissa in the penalty area and Romelu Lukaku struck through the middle from the spot to double Napoli's lead just after half-time.
Teams led by Antonio Conte are notoriously difficult to break, but Fiorentina should have notched one back in a remarkable passage of play that featured a stunning save from Alex Meret and goal-saving blocks from both Amir Rrahmani and Olivera.
Once again the hosts' waste proved costly, and Scott McTominay made it three by pounced on Pietro Comuzzo's poor clearance from Anguissa's cross and smashed into the bottom left.
Fiorentina then seemed completely dejected and Napoli were allowed to play out the last twenty minutes with relative ease.
The absence of Atalanta and Inter Milan, who are in Riyadh for the Supercoppa Italiana, means Napoli top the table but have played at least one more match than their two closest title rivals.
Fiorentina, meanwhile, are in danger of dropping out of the top six altogether, having been in third place in November, and will look to their next match against 20th-placed Monza to end their longest streak without a win since September 2022.
Flashscore Man of the Match: Amir Rrahmani (Napoli)
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