The Spanish outfit dominated proceedings in the first period, but stern defending from Milan restricted their ability to create clear chances.
It was Milan that took a shock lead in the second period, however, when Kevin-Prince Boateng found the bottom corner from close range.
Read how it all unfolded in Sports Mole's minute-by-minute live commentary of the action below.
MILAN: Abbiati; Abate, Mexes, Zapata, Constant; Montolivo, Ambrosini, Muntari; Boateng, Pazzini, El Shaaraway
BARCELONA: Valdes; Alves, Puyol, Pique, Alba; Fabregas, Xavi, Busquets; Iniesta, Messi, Pedro