Walter Samuel lit the touchpaper on the fieriest of Milan derbies, and stood back as 10-man Inter Milan rode out a controversial 90 minutes to heap further woe on Massimiliano Allegri with a 1-0 Serie A win.
All eyes had been on Inter striker Antonio Cassano, who had switched sides at the San Siro before the start of the Serie A season, but it was veteran centre-back Samuel who won the game for the Nerazzurri with a headed goal in the third minute.
Riccardo Montolivo was denied the chance to restore parity courtesy of a controversial call from referee Paolo Valeri, but AC Milan only had itself to blame for failing to capitalise when Yuto Nagatomo's dismissal put Inter on the back foot for much of the second half.
Juventus and Napoli remain locked together at the top after winning their respective games.
Juventus picked up a hard-earned 2-1 triumph at Siena.
Andrea Pirlo gave the Bianconeri the lead with an early free-kick, but the Old Lady was pegged back in first-half stoppage-time by Emanuele Calaio's equaliser.
However, the visitor refused to settle for the draw and secured all three points five minutes from time when Claudio Marchisio volleyed home the winner.
Napoli also had to do it tough against Udinese at the Stadio San Paolo, with
Roma bounced back from its 4-1 defeat to Juventus a week ago to pick up all three points at the expense of Atalanta in a 2-0 win at the Stadio Olimpico.
Pescara's mini-revival was ended in emphatic fashion as Lazio ran out a comfortable winner 3-0, with a Hernanes free-kick and a brace by