Diego Milito put the Nerazzurri ahead from the penalty spot in the 17th minute and Antonio Cassano made it two 11 minutes before the interval.
But a goal from Romulo only six minutes later gave the Viola hope and they continued to pile the pressure on their hosts.
A red card for Gonzalo Rodriguez in the 62nd minute diminished Fiorentina's chances of getting an equaliser but Inter still had to work hard for the three points, which take them above Sampdoria and into a share of third place with Lazio, four points behind league leaders Juventus.
But joining La Vecchia at the top of table were Napoli, as Edinson Cavani's second-half penalty earned a deserved 1-0 victory over Sampdoria at the Stadio Luigi Ferraris.
The Uruguay international converted after a foul which earned Daniele Gastaldello a red card and Napoli could easily have won by more but for wayward finishing throughout.
Torino took full advantage of a spectacular collapse from Atalanta to claim a 5-1 victory - and a first win on the road this season.
German Denis ensured a steady start for the hosts before Rolando Bianchi equalised from the penalty spot before half-time.
Alessandro Gazzi, Alen Stefanovic and Danilo D'Ambrosio all notched after the break, Bianchi completing the rout with his fourth goal of the season.
Elsewhere, Bologna rediscovered some of their early-season form with a comprehensive 4-0 victory over 10-man Catania
Stefano Pioli's men had been finding goals hard to come by but Tiberio Guarente got the Rossoblu rolling once again with a cool first-half finish.
Alberto Gilardino registered a goal either side of the break with Panagiotis Kone completing the rout following Ciro Capuano's late dismissal.
Cagliari's recent woes continued as they slumped to a 2-1 home defeat against Pescara at a scarcely-populated Stadio Is Arenas.
Goals from Christian Terlizzi and Vladimir Weiss lifted Pescara to a second win in five days and heaped further misery on Cagliari, who replied late on through a Mauricio Pinilla penalty but also lost Luca Rossettini to a red card.
The Sardinian club played in front of around 5,000 season-ticket holders today after being permitted to open just one section of the ground due to ongoing safety concerns.
Fabrizio Miccoli hit a hat-trick as Palermo swept Chievo aside 4-1 in Sicily, heaping more pressure on visiting coach Domenico Di Carlo.
Di Carlo had received a vote of confidence from the club's sporting director Giovanni Sartori during the week but his players did nothing to further enhance his job security with a limp display.
In contrast, it was a first win in charge for new Palermo boss Gian Piero Gasperini following two defeats - and he will be toasting Miccoli.
The veteran striker opened the scoring after 13 minutes and restored the hosts' advantage after Marco Rigoni equalised. Luigi Giorgi made it 3-1 with 10 minutes to go before Miccoli completed his treble shortly afterwards.
Meanwhile, Lazio struck twice in the first half to record a 2-1 victory which leaves them four points off the summit.
Cristian Ledesma set up the first for Ederson and then scored himself from the penalty spot to give the hosts a commanding half-time lead.
Massimo Paci's stoppage-time goal was all Siena had to show for their efforts and they remain in the bottom three after six games.
And in the early game, Udinese and Genoa picked up a point apiece in a goalless draw played at a pedestrian pace at the Stadio Friuli.
Sunday's Serie A Results
Saturday's Serie A Results