A modest Milan side, beaten at home by Tottenham Hotspur in the Champions League two weeks ago, would not have had a hope of winning the title in recent years when a dominant Inter Milan were racking up five in a row.
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Massimiliano Allegri's team are only prospering now because of a lack of real rivals and injury problems at Inter, who have also had to deal with Jose Mourinho and Rafa Benitez departing before ex-Milan boss Leonardo came in to steady the ship.
Second-placed Inter may yet snatch a sixth straight title with 11 games left given they must play Milan who have had a few shaky results this season.
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"Five points ahead of second is a good lead but the championship still has a long way to run," Allegri, plucked from relative obscurity at Cagliari to be Milan boss, told Sky.
"I'm happy with what the lads did. The first half we were good in defence and midfield but we made lots of mistakes up front."
The fact that third-placed Napoli, who went bankrupt seven years ago before working their way back up the league pyramid, are in the title hunt is further proof of Serie A's inherent weakness.
Monday's game had more misplaced passes than Barcelona might manage in a season in Spain and Italy's loss of their fourth Champions League qualifying place for 2012 to Germany last week was sobering for Serie A which still boasts the European champions in Inter.
Napoli never turned up and although rain made play tricky and Zlatan Ibrahimovic's penalty which gave Milan a 49th-minute lead was dubious, the visitors can have no complaints after Kevin Prince Boateng and Alexandre Pato sealed the win.
"We didn't play like we know we can. We lost our way after going behind and everyone just went forward," Napoli boss Walter Mazzarri said.
"I struggle to think it was a penalty," he added in reference to Salvatore Aronica's handball as he went to block off Pato.
