Kevin De Bruyne put Napoli ahead from the penalty spot before deadline-day signing Rasmus Hojlund marked his debut with a calm finish. Dutch defender Sam Beukema, also making his Serie A bow, added a third early in the second half.
Luca Ranieri gave Fiorentina hope with a late header, forcing David de Gea into several saves as the hosts pushed to get back into the contest. Conte praised the performance overall but demanded greater control in key moments.
"We showed from the start that we are ready and played with the right attitude," Conte said. "It is not possible that after dominating the match, we end up struggling in the final minutes. When you play this well, you have to kill the game and close it out."
Much of the focus was on Hojlund, who joined Napoli on loan from Manchester United on deadline day, with an option to buy for 44 million euros. The 22-year-old striker impressed on his debut, despite not being expected to start so soon after his arrival.
"Hojlund is only 22, but he has great qualities and potential," Conte said. "We brought him from Manchester United, as we did with McTominay, and it has brought us luck again. He is a lad with excellent prospects."
Napoli midfielder Frank Anguissa also caught the eye, continuing his fine run of form after being named Serie A Player of the Month for August. The Cameroonian international set up Beukema's goal and remains one of only three midfielders this season, alongside Nico Paz and Mario Pasalic, to have both scored and assisted in the league.
Napoli's third consecutive victory leaves them joint-top of Serie A with Juventus on nine points, with Conte's side showing early signs of being serious title contenders.
Mainz 05 manager Bo Henriksen praised his team's determination after a 2-1 UEFA Conference League victory over ACF Fiorentina, secured by a stoppage-time header from Jae-Sung Lee.
Luciano Spalletti's arrival at Juventus on October 30, has brought renewed belief and direction to Turin, with the veteran coach already stamping his authority after replacing Igor Tudor.
Inter Milan reignited their Champions League charge on Wednesday night as Marcus Thuram made a long-awaited return from injury to inspire a tense 2-1 victory over Kairat Almaty at San Siro.
Napoli's attacking woes deepened as Antonio Conte's side were held to a 0–0 draw by Eintracht Frankfurt in the Champions League, leaving both clubs on four points after four games.
