Messi powers Barca into last-eight
Two goals down from the first leg, Messi was on target after just five minutes and although Christian Abbiati was at his best to repel the home side, he could do little to stop Messi doubling Barca's lead five minutes before the break.
David Villa - preferred by Jordi Roura to Cesc Fabregas - scored a vital third before Jordi Alba's injury time strike settled the tie 4-2 in Barcelona's favour.
Milan struggled to deal with the movement of an inspired Barcelona while they failed to create enough chances with Victor Valdes rarely tested in the home goal. It took just five minutes for Barcelona to reduce the deficit and take the lead on the night.
Xavi's intricate pass found Messi whose unstoppable curling effort into the top corner gave Abbiati no chance. It was Messi's 52nd goal of the season and he almost created one for Andres Iniesta moments later before Philippe Mexes intercepted.
The home side were in full flow and after 13 minutes Abbiati was forced into making a fantastic save to tip Iniesta's effort on to the bar before Messi somehow shot wide following up.
Milan's best chance fell to lone striker M'Baye Niang who broke the offside trap and although he beat Valdes, his effort hit the post and away from danger. Milan were made to pay just seconds later, as Massimo Ambrosini lost out to Iniesta who fed Messi, who looked to be in an offside position when the ball was played.
The flag stayed down and the Argentinian cut inside and fired past a helpless Abbiati with his lethal left foot. Barca began the second period much like the first and after 55 minutes Xavi's pin-point pass eluded a sliding Cristian Zapata to find Villa, whose left-footed finish gave the Milan stopper no chance.
In a bid to increase their attacking options, Massimiliano Allegri brought on Robinho and Sulley Muntari for Niang and Ambrosini respectively. With 15 minutes left goalscorer Villa was replaced by Alexis Sanchez, with Carlos Puyol coming on for Javier Mascherano shortly afterwards.
The home side were thankful for Jordi Alba's intervention after 81 minutes to deny Robinho a simple chance from close range. However, Barca's win was sealed late on when Alba burst down the left to fire past Abbiati.
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