Kevin-Prince Boateng’s controversial opening goal set the Italians on their way and Sulley Muntari’s superb second put Massimiliano Allegri’s side firmly in the driving seat ahead of Mar 12’s return leg at the Nou Camp.
Muntari flashed an early shot over the bar from long range for the hosts. But they were alerted to the Spanish giant’s threat shortly after when Philippe Mexes was forced to make a vital clearance to prevent Jordi Alba’s cross reaching Pedro Rodriguez and though the same defender then handled Dani Alves’ delivery, Pedro had been flagged offside.
Mexes collected the first yellow card of the night for tripping Lionel Messi.
The second-half was a more livlier affair which saw Sergio Busquets booked for the visitors early on after fouling Giampaolo Pazzini, moments before the controversial opening goal.
Montolivo’s shot took two deflections, the second off the raised arm of Cristian Zapata, before Boateng fired low past Victor Valdes. Gerard Pique was booked for protesting and was later replaced.
Messi tried to bring Barcelona back into it blazed an uncharacteristically wayward free-kick high into the stand before Andres Iniesta almost charged down Abbiati’s clearance in the six-yard box.
Valdes saved Pazzini’s acrobatic volley but at the other end, Iniesta fizzed a drive a foot or so wide and Xavi curled a free-kick just over.
But it was Milan who found a second goal with nine minutes remaining as substitute M’Baye Niang and El Shaarawy combined to set up Muntari, who gave their beautiful approach play the classy finish it deserved.
Carles Puyol, sporting a huge sticking plaster on his forehead after a clash with Pazzini, headed Xavi’s corner wide as Barça sought an important away goal.
But it was in vain, with Milan in fact having the best chance during five minutes of stoppage time as late substitute Bakaye Traore wasted a three-on-two break.
AC MILAN: Abbiati, Abate, Mexes, Zapata, Constant, Ambrosini, Montolivo, Muntari, Boateng, Pazzini (Niang 75), El Shaarawy (Traore 88).
BARCELONA: Valdes, Dani Alves, Pique, Puyol (Mascherano 88), Jordi Alba, Xavi, Busquets, Fabregas (Sanchez 62),Pedro, Messi, Iniesta.
Referee: Craig Thomson (Scotland).