AC Milan rose to second after substitute Klaas Jan Huntelaar netted his first two goals for the club deep into injury time to seal a 2-0 win at Catania.
The Dutch striker has floundered since a close season move from Real Madrid but he finally showed his worth by powering in Filippo Inzaghi's layoff and then producing a perfect chip.
Juventus, who host Inter next Saturday, slipped to third after losing 2-0 at Cagliari despite dominating for spells.
Argentina striker Milito converted Inter's spot-kick after being brought down, having earlier missed an easy chance.
Champions Inter, chasing their fifth straight scudetto, thought they had gone ahead early in the second half when Walter Samuel's header was controversially ruled out for shirt tugging.
Alberto Gilardino hit the post in Fiorentina's only real chance and Inter forward Samuel Eto'o's shaky form continued when he wasted a late one-on-one with goalkeeper Sebastien Frey.
"I have just one duty - to be honest. I don't need to win anything, not the championship, not the Champions League," Inter coach Jose Mourinho told Sky television when asked if his side must now focus more on Europe with Serie A becoming a stroll.
"My team won and I am good again," he joked, referring to criticism after the midweek Champions League loss at Barcelona.
PENALTY APPEAL
Juve, beaten at Bordeaux in the Champions League on Wednesday, have struggled to find fluency this term under rookie coach Ciro Ferrara but were slightly unfortunate in Sardinia.
Nene opened the scoring with a rocket from just outside the edge of the area on half an hour and after sustained Juve pressure, including a strong penalty appeal, Alessandro Matri broke at the other end to seal victory for Cagliari late on.
Promoted Parma moved up to joint fourth with Sampdoria, beaten 3-0 by sixth-placed Genoa in Saturday's derby, thanks to a 1-1 home draw with Napoli.
Cagliari are seventh with Bari, AS Roma and Fiorentina just a point further back after 14 games.
Bari, who came up last season and have rejected two recent takeover attempts, came from behind to win 2-1 at home to bottom club Siena while Roma continued to improve with their own 2-1 comeback victory at lowly Atalanta.
Lazio's poor season drags on after a 0-0 draw at home to Bologna with their former coach Delio Rossi losing in his first league game in charge of Palermo, beaten 1-0 at Chievo.
