Inter thumped struggling Frosinone 4-0 at San Siro on Sunday to jump up from fourth place to top spot in Serie A.
A fourth straight win for Roberto Mancini's side without conceding a goal came courtesy of four different goalscorers.
The hosts were in charge right from the start and eventually turned first-half dominance into the lead when Jonathan Biabiany fired home on the rebound.
All eight of Inter's league wins this term had been by a single goal, but Mauro Icardi's well-worked effort after the break gave them an unusually healthy advantage.
And the result was capped late on as Jeison Murillo bundled home his first Inter goal, before Marcelo Brozovic found the net in stoppage time.
Fiorentina had gone into the weekend as leaders, but they drop down to third despite battling back from two goals down to draw 2-2 at home to Empoli.
Marko Livaja finished emphatically to give the visitors a lead, although it appeared he was in an offside position when Riccardo Saponara released his pass.
An excellent strike from Marcel Buchel had Paulo Sousa's men under serious pressure going into half-time, but substitute Nikola Kalinic scored twice – with a header and a close-range finish – in the space of six second-half minutes.
Kalinic almost made it a hat-trick to secure a crucial victory, but his late shot cannoned off the crossbar.
Napoli are second after they comfortably won 2-0 at Hellas Verona, although it took 67 minutes for their dominance to be converted to goals.
Lorenzo Insigne opened the scoring before setting up Gonzalo Higuain to wrap up the points six minutes later as Maurizio Sarri's side made it eight wins in 10 league matches.
The top two will clash next Monday when Napoli host Inter.
Lazio drew for the first time this season after Antonio Candreva's penalty saw them held 1-1 at home to Palermo, who had taken the lead through Edoardo Goldaniga.
Vincenzo Montella's first match in charge of Sampdoria ended in a 1-0 defeat at Udinese. Emmanuel Agyeman-Badu scored the winner and Ervin Zukanovic was sent off for the visitors in stoppage time.
Genoa handed Sassuolo only their third defeat of the season, winning 2-1 in a match that saw Leonardo Pavoletti head in a stoppage-time winner only seconds after the visitors had equalised.
Diego Perotti and Domenico Berardi had been sent off in the first half in the same incident after the former Sevilla man had taken issue with the young striker kicking out.
Bottom-of-the-table Carpi lost 2-1 at home to Chievo, while Torino won 1-0 at Atalanta to move above their opponents into ninth.
