Costa, whose late header earned Atletico a 1-0 first-leg win at the San Siro, acrobatically put the Spanish side ahead with the first chance of last night’s clash. Koke floated in a cross from the edge of the area and Costa, having peeled away from Milan centre-back Adil Rami, flung himself at the ball and managed to volley it past Christian Abbiati.
Milan got a foothold in the tie in the 27th minute when Kaka headed home from their first decent opportunity. Mario Balotelli played the ball out to Andrea Poli and his deep cross was met at the far post by Kaka, whose header flew into the net via the thigh of Atletico defender Juanfran.
Atletico punished Milan four minutes later, albeit with the help of a huge slice of good fortune.
Costa chested the ball down to Arda on the edge of the area and the Turkey midfielder’s half-volley took a crucial deflection off Rami before looping into the net.
The momentum was back with the Spanish side now and Garcia almost made it 3-1 with a spectacular overhead kick just before the break, the ball going inches wide.
Atletico were back on the attack after the interval with recent Spain debutant Costa and Gabi going close before Milan substitute Robinho – another Real Madrid old boy – drew a parry out of Thibaut Courtois with a cross-shot.
Milan were pushing but they could not find a breakthrough before Atletico all but settled the tie in the 71st minute when Garcia climbed above two visiting defenders to head home Gabi’s free-kick.
There was no way back for Clarence Seedorf’s men after that, although Robinho did hit the crossbar soon after, and Atletico wrapped up a convincing win when Costa drilled a shot in off the far post in the 85th minute.
Atletico are were one of only two sides still unbeaten in this season’s competition — the other being their cross-city rivals, Real Madrid.
ATLETICO MADRID: Courtois, Juanfran, Miranda, Godin, Filipe Luis, Turan (Rodriguez 78), Gabi, Mario Suarez, Koke (Diego 81), Raul Garcia (Sosa 72), Diego Costa.
AC MILAN: Abbiati, Abate, Rami, Bonera, Emanuelson, Essien (Pazzini 68), Poli, De Jong (Muntari 78), Kaka, Taarabt (Robinho 46), Balotelli.
Ref: M Clattenburg (England).