One week after he had sunk his former teammates at Inter Milan Ibrahimovic once again proved the match winner.
On 45 minutes, with his back to goal, the Swede juggled the ball and executed an acrobatic bicycle kick which totally flummoxed Fiorentina keeper Artur Boruc, the ball thudding into the top far corner.
Ibrahimovic then thumped his chest so hard in celebration he appeared to have injured his arm, but he duly emerged intact for the second half.
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AC Milan thought they'd won a penalty in the 23rd minute after Per Kroldrup collided with Mathieu Flamini in the box, but the referee turned down their appeals.
On the half hour Boruc, standing in for long term injury victim Sebastien Frey, two-fisted Clarence Seedorf's shot out of danger.
With coach Massimiliano Allegri fielding his three-pointed midfield formation of Gennaro Gattuso, Massimo Ambrosini and Flamini, Milan, the goal aside, never produced dream football but were nevertheless solid and good for the three points.
They maintained their dominance after the break, and should have doubled their lead when Alessandro Nesta failed to convert a free header from eight yards, holding out to move onto 29 points.
Fiorentina finished a man down after Kroldrup was sent off after picking up one yellow card for a foul and then a second for having a verbal joust with referee Antonio Damato seconds from the finish.
Lazio are placed second to Milan on 25 points but can cut the deficit to one point if they get a result at Parma on Sunday.
"We're improving week by week," Allegri noted.
"I'm very happy with our progress."
He then singled out for special mention Daniele Bonera, and Flamini.
"Flamini's very good, Robinho too was good...I'm contented."
Turning to Ibrahimovic's moment of magic Allegri added: "That's par for the course for him to do that! He has other qualities rather than just physical..I don't know what Milan would be without him."
Earlier, a Jeremy Menez-inspired Roma beat Udinese 2-0 to move into provisional third place ahead of the remainder of the weekend's action.
Menez scored a superb solo goal in the 24th minute before setting up Marco Borriello for Roma's second in the 56th minute to wrap up the points for the capital club.
The France international's key interventions enabled Roma to notch up their fifth victory from their last seven games in the Italian top flight.
Roma's reward was to move up three places into third, on 22 points.
Sunday's programme includes champions Inter Milan, languishing in fifth place, at Chievo.
Italian press reports have claimed that new manager Rafael Benitez has only two matches to save his Inter Milan career.
Inter though will be without nine key personnel - Coutinho, Maicon, Cristian Chivu, Walter Samuel, Thiaggo Motta, Sulley Muntari, McDonald Mariga, Joel Obi Chukwuma, Diego Milito, and keeper Julio Cesar.