Second-placed Roma came from behind to beat city rivals Lazio 3-2 last night and Alberto Gilardino scored twice to help Milan rediscover their form with a 5-0 victory at Sampdoria.
All of Serie A's top sides won, with David Trezeguet netting his second hat-trick of the campaign in Juve's 3-0 victory over Empoli while leaders Inter beat Genoa 4-1.
Fiorentina edged out Napoli 1-0 thanks to Christian Vieri's second-half strike from Adrian Mutu's low cross.
Inter have 24 points from 10 games, Roma are three points behind while Juventus and Fiorentina are four adrift. Mid-table Milan moved above Sampdoria after a third league win this season.
Roma, without injured skipper Francesco Totti, went behind in the 129th Rome league derby when Lazio striker Tommaso Rocchi took advantage of slack defending to score after 12 minutes.
Struggling Lazio, who won 3-0 and drew 0-0 in last season's derbies, were brought back to reality by Roma's superb passing which enabled Mirko Vucinic to net his third in three games.
Brazilian midfielder Mancini shone all night and grabbed Roma's second after a neat one-two with Philippe Mexes before Simone Perrotta scored the third following Vucinic's layoff.
Cristian Ledesma's expert free-kick off the bar made for a nervous finish but Roma deservedly claimed the bragging rights.
Milan's tortuous season received a big boost at Sampdoria after consecutive home defeats to Empoli and Roma.
Striker Gilardino was the architect, setting up Kaka for a simple finish and then heading in the second before an excellent tight turn in the area allowed the 25-year-old to make it 3-0.
Yoann Gourcuff and Clarence Seedorf added the other goals as the European champions roared back to their best on a ground where they began the campaign with a 3-0 win over Genoa.
David Trezeguet also repeated his opening day feat by bagging a hat-trick for suspension-hit Juventus, whose main absentee was Pavel Nedved. Despite the problems, Claudio Ranieri left Alessandro Del Piero on the bench with an eye on Sunday's big match with Inter.
France striker Trezeguet netted his first from the spot after being brought down early in the second half. The league's top scorer then rose to head in a free-kick at the near post to make it 2-0 before scoring his 10th of the season.
Champions Inter went ahead when defender Ivan Cordoba nodded home after Hernan Crespo's header came off the bar.
Esteban Cambiasso doubled their advantage after 50 minutes when he reacted quickest when Luis Figo's free-kick was spilled before Genoa pulled one back through Abdoulay Konko.
Inter's Honduran striker David Suazo exposed some poor defending to score almost immediately and then won a penalty which Julio Cruz converted late on.
Boss Roberto Mancini was also thinking of Sunday's trip to Juve and kept Zlatan Ibrahimovic and Walter Samuel on the bench to avoid them picking up bookings and being banned.