Milan beat Napoli 5-2 last night to record their first league win at the San Siro this term with Pato scoring on his debut and Ronaldo netting twice to open his season's account.
Earlier, Inter maintained their seven-point lead at the top of Serie A after Zlatan Ibrahimovic's double helped the champions to a 3-2 win at Siena.
Roma moved back up to second but had to come from behind to beat Atalanta 2-1 thanks to Francesco Totti's low free-kick and Mancini's thumping strike.
At San Siro, Ronaldo, playing in only his second match this season after injury problems, gave Milan a 15th-minute lead when his shot ballooned off Napoli keeper Gennaro Iezzo and just crossed line before Andrea Cupi hooked clear.
Napoli equalised when Ezequiel Lavezzi's cross was toe-poked in by Roberto Sosa, but Milan were soon back in front when Iezzo denied Brazilian teenager Pato twice only for Clarence Seedorf to smash home.
Maurizio Domizzi pulled Napoli level from the penalty spot after 38 minutes of an enthralling game, which Milan wrapped up in the second period through Ronaldo's diving header, Kaka's crisp shot and Pato's cool finish. Napoli had Walter Gargano sent off in stoppage time.
Milan remain 12th in the table but have three games in hand on most clubs.
Roma went above Juventus, who drew 1-1 at Catania on Saturday and are now three points behind in third, while Udinese stayed fourth after Fabio Quagliarella secured a 1-0 win at bottom side Cagliari.
Fiorentina are a point further back in fifth after Adrian Mutu's brace set up a 2-1 win at Parma, although his second from the penalty spot late on followed a dive from Christian Vieri.
Antonio Cassano scored one and was again influential in Sampdoria's 3-0 home win over Palermo, their main rivals for a UEFA Cup spot. Lazio's poor season continued when they were unlucky to lose 2-1 at home to Genoa.
Returning from a three-week mid-season break, unbeaten Inter would not have expected such a tough match at lowly Siena, where they clinched the title last season.
Ibrahimovic converted a debatable penalty on 26 minutes when Julio Cruz collided with Paul Codrea.
Siena equalised quickly when Inter defender Ivan Cordoba appeared to get the last touch under pressure from Massimo Maccarone but the champions went back in front on the stroke of half-time when Esteban Cambiasso slid in Ibrahimovic's pass.
The Swede grabbed his 11th of the league campaign in the second half with a cracking shot and despite Fernando Forestieri's late goal, Inter hung on for their seventh straight Serie A win and 11th successive victory in all competitions.