A superb header by Ronaldinho spurred AC Milan to a 1-0 over Inter Milan Sunday and hand the Serie A champions their first defeat under new coach Jose Mourinho.
The Brazilian soared in the area and powerfully headed home a cross from his compatriot Kaka to settle the derby in the 37th minute.
Inter had defender Nicolas Burdisso sent off for a second yellow in the 77th and Marco Materazzi, who had been substituted, was also given a red card for dissent from the bench during the closing stages.
Inter have 10 points from five games, two behind surprise leaders Lazio, who won 3-1 at Torino earlier Sunday.
Milan have nine after recovering from a slow start with their third consecutive league win.
Ronaldinho, in the starting lineup after being on the bench in Milan's back-to-back league wins over Lazio and Reggina, had come close in the middle of the first half with a snap shot but Inter keeper Julio Cesar produced a fine reflex save.
Milan also had a penalty appeal for a shove on Kaka in the area turned down.
Milan looked the sharper side after the break and Mourinho tried to shake his men up by bringing on strikers Adriano and Julio Cruz.
Chances opened up at both ends after Burdisso's expulsion.
Adriano wasted Inter's best chance to equalise deep into stoppage time, heading wide when unmarked at the far post.
In Turin, Goran Pandev struck on the half hour to set Lazio on their way and Argentine forward Mauro Zarate netted twice in the second-half, including a late penalty, to wrap things up and take his goal tally to six.
Torino, who finished the match with nine men after keeper Matteo Sereni was sent off for fouling Cristian Brocchi to concede the penalty and Francesco Pratali went for a second yellow, pulled one back with a stoppage-time Nicola Amoruso penalty.
Napoli are second with 11 points after they won 1-0 at Bologna with a German Denis header three minutes from time.
Last year's runners-up AS Roma gave their stuttering campaign a boost with a 2-0 home win over in-form Atalanta thanks to first-half goals by Christian Panucci and Mirko Vucinic.