Milan's Scudetto hopes took another sizeable dent as they slumped to their second 1-0 home defeat in succession after an insipid performance against a spirited Roma side, while Genoa and Fiorentina ground out a bore-draw at the Stadio Luigi Ferraris.
Roma talisman Francesco Totti missed out through injury, but it was his replacement Mirko Vucinic who sealed the points with a bullet header midway through the second half.
Roma could have doubled their advantage shortly afterwards after Massimo Ambrosini was sent off for a foul in the box, but Daniele De Rossi contrived to chip the resultant penalty onto the roof of the net.
The home side enjoyed plenty of possession in the opening stages, and France centre half Philippe Mexes had to produce a well-timed tackle to deny Kaka after a quarter-of-an-hour.
Moments later Kaka had another sight of goal after Alberto Gilardino's dummy, but he dragged his shot across goal and wide.
Thereafter Roma began to exploit the huge spaces Milan were leaving at the back, and Brazilian right back Cicinho was at the heart of much of their attacking play.
First he forced a good save from Dida with a low, right-footed shot across goal, and from the resulting corner his well-struck volley drew another acrobatic save from his international colleague.
Vucinic spurned a great chance to give Roma the lead just before the half-hour when he side-footed wide with only Dida to beat, but as the half progressed Milan gradually began to assert themselves.
Stand-in Roma captain De Rossi produced a terrific block to deny Clarence Seedorf, and when Seedorf fed Kaka on the left, Mexes just managed to deflect his curling shot out for a corner.
Milan upped the tempo in the early stages of the second half, with Gennaro Gattuso and then Alberto Gilardino both testing Doni in the Roma goal. But Roma weathered the storm, and after 72 minutes they were in front.
Mancini found Cicinho on the right, and he had the time to guide a perfect cross onto the head of Vucinic, who buried the ball past Dida at the near post.
Roma could have made the game safe when the referee awarded a penalty and showed Ambrosini a second yellow card for a slight tug on De Rossi.
The Italy international got up to take the penalty himself, but his casual chipped effort was just over-hit.
Milan coach Carlo Ancelotti threw on left-sided Brazilian Serginho in search of an equaliser, but while his crosses proved a constant source of danger, Roma held firm for a deserved three points.
Genoa 0-0 Fiorentina
Chances were thin on the ground in this insipid encounter, although Genoa had a strong penalty appeal turned down when Per Kroldrup brought Julio Cesar de Leon down in the second half.
Fiorentina went close through Romania forward Adrian Mutu and striker partner Giampaolo Pazzani, while Genoa's Leon thought he should have had another penalty in the first half, although Alessandro Gamberini did not appear to foul the theatrical Brazilian midfielder following his exciting run, who was booked at half time for continuing his protests.
Leon was the main threat in the match, forcing a good save from Sebastien Frey in the 13th minute with a long-range free kick and having a shot deflected wide a few minutes after his first penalty claim.
Frey made another good stop soon afterwards when Tomas Ujfalusi gave the ball away and Ivan Juric found an unmarked Giuseppe Sculli in the box, his shot well-blocked by the former Parma shot-stopper.
Pazzini had a good chance for the visitors on 67 minutes, poking wide after good work from Mutu and Fabio Liverani, but neither side could really make a breakthrough. Fiorentina remain unbeaten and move up one spot to third and Genoa move up to ninth.
