Internazionale piled the pressure on city rivals Milan as Walter Samuel's early header was enough to hand them victory at the San Siro.
With both heavyweights having endured difficult starts to the season as reigning champions Juventus have again set the pace, a fourth defeat in seven Serie A matches left manager Massimiliano Allegri's Milan way down in 11th place.
Despite an encouraging victory away at big-spending Zenit St Petersburg on Wednesday, even the dismissal of Japanese midfielder Yuto Nagamoto just three minutes into the second half failed to spark a response from Milan. Inter's players celebrated the victory – their third in successive derby meetings – at the final whistle as if they had regained the title they surrendered to Juventus last year but they will need to improve considerably if coach Andrea Stramaccioni is to lead his team to the Scudetto.
In a bad-tempered match that saw the hosts have a goal disallowed and a late penalty appeal waved away as they chased an equaliser, Samuel's fourth-minute header following a howler from goalkeeper Christian Abbiati came after a rapid start had seemed to surprise Milan. They slowly improved and saw Riccardo Montolivo's goal disallowed for a foul on Inter goalkeeper Samir Handanovic.
Nagatomo's dismissal for a second yellow card shortly after the break allowed Milan to lay siege to the Inter goal but they could not find the equaliser, despite going agonisingly close on several occasions. They were also denied what looked to be a clear-cut penalty late on when Robinho appeared to be brought down by Samuel.
In a heated atmosphere, there were several calls over the speaker system for the fans to stay in their seats and behave themselves or risk the game being suspended. Former Milan striker Antonio Cassano was roundly jeered throughout by the home support in his first match against his old club since his switch to their bitter rivals in the summer.