Faltering leaders Inter Milan drew 1-1 at Palermo on Saturday to give second-placed Milan a glimpse of the summit for the first time this term but instead they lie one point behind.
Napoli, who have slipped to seventh after a poor run, produced waves of attacks in the first half and went ahead thanks to a defensive calamity from Milan's Massimo Oddo.
The right-back collided with his own goalkeeper Christian Abbiati and left Napoli defender Hugo Campagnaro to slot Ezequiel Lavezzi's pass into an open goal on 13 minutes.
The day got immediately worse for Milan, already hampered by a string of injuries, when forward Alexandre Pato limped off on what was meant to be his comeback from a thigh problem.
Napoli could easily have doubled their advantage before Ronaldinho, celebrating his 30th birthday, revived Milan as he has done so often this season by floating over a cross which Filippo Inzaghi headed in for a 26th-minute equaliser.
Leonardo's side pushed for a winner in the second half but winger Mancini spurned the best chance as the Rossoneri again showed they can lack a clinical edge.
"We knew that playing against Napoli is never easy," Ronaldinho told Sky TV. "We are happy to continue like this, at least we are nearer top spot. We created some chances."
RECENT RENAISSANCE
AS Roma, in third, beat Udinese 4-2 on Saturday to cut the gap on Inter to four points and liven up the title race.
Nine games remain and on Wednesday Milan visit Parma, who drew 1-1 at Bari on Sunday to slow their recent renaissance.
Sampdoria climbed level on points with fourth-placed Palermo in the race for the final Champions League qualifying spot after the returning Antonio Cassano embarrassed Juventus reserve goalkeeper Antonio Chimenti from 40 metres in a 1-0 home win.
The forward, whose great start to the season was halted by injury and loss of form, burst back into the limelight on 77 minutes and could have scored two more against a Juve side enduring a dreadful campaign.
Sixth-placed Juve had lots of possession but continue to lack width since Pavel Nedved's retirement as interim coach Alberto Zaccheroni employed yet another formation following their midweek humiliation at Fulham in the Europa League.
Fourth-bottom Lazio opened up a four-point cushion above the drop zone after forwards Tommaso Rocchi and Sergio Floccari scored in a 2-0 win at Cagliari, whose fans will now be given refunds for the defeat under a club initiative.
Atalanta moved up to third bottom and the final relegation spot with a 3-0 victory at home to Livorno, who now prop up the league ahead of Wednesday's trip to Inter.
Second-bottom Siena are level on points with Atalanta after a 1-0 win over Bologna. Mid-table Chievo drew 1-1 with Catania.
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