Messi, Barcelona advance to CL semis
Messi opened the scoring in the 11th minute at the Camp Nou before Antonio Nocerino countered with a 32nd-minute strike that gave the Italian champions a valuable away goal after the first leg had ended scoreless.
Barcelona's attack did not relent and the defending champions were awarded a second penalty when Alessandro Nesta needlessly yanked Sergio Busquets down inside the area during a corner, and Messi calmly scored his record 14th goal of the competition this season.
Goalkeeper Christian Abbiati kept Milan in reach but could not stop Andres Iniesta from completing the victory from close range in the 53rd.
Messi's two goals took his season tally to 58 and set a record for goals in a Champions League campaign with 14. Barcelona also became the first club in the modern-day edition of the European Cup to reach five straight semifinals. Only Real Madrid had accomplished the feat en route to winning five straight trophies between 1956-60.
Milan, meanwhile, hasn't reached the last four since winning its last title in 2007 and its players quickly surrounded Dutch referee Bjorn Kuipers to protest after the final whistle.
Zlatan Ibrahimovic set up the equalizer and had a penalty appeal turned down in the second half, but was mostly muted by defender Gerard Pique in his first return to the Camp Nou since leaving Barcelona in 2010.
The match began with an electrifying pace as Barcelona pushed forward from the start and managed 12 first-half shots, with Iniesta failing to finish off a nice combination with Messi, who himself shot straight at Abbiati following an upfield charge after five minutes.
Messi uncharacteristically shot wide across the face of goal after displaying great control with just Abbiati to beat, but scored his opener moments later. Luca Antonini slid into Messi from behind during a scramble in the area, and the 24-year-old Argentina forward became the youngest player to reach 50 goals in the competition when he slotted his spot kick just out of reach of a diving Abbiati.
While Barcelona maintained control with its usual passing game, the Rossoneri defense held firm to avoid further goals before Robinho's individual effort created the equalizer. The Brazil striker drew a number of players before spotting Ibrahimovic, who put Nocerino through on goal to slot a shot past Victor Valdes from the right side of the area.
Knowing a scoring draw would put Milan through, Barcelona kept pressing for another goal and Abbiati had to stretch to push away a shot from Xavi Hernandez, a surprise starter despite a calf injury.
Instead it was Milan's disciplined defense - which held Barcelona scoreless in a Champions League game for the first time in more than two years in the first leg - that finally cracked as Nesta gave away a cheap penalty when he tussled with Busquets during a corner.
Messi sent Abbiati the wrong way from the spot to match Mazzola Altafini's 49-year mark of 14 goals in a European Cup campaign. Ruud van Nistelrooy scored 13 in 2002-03, a mark that Messi matched last year.
Milan clung on after the restart but were unlucky when Messi's effort was deflected into the path of Iniesta, who picked his spot inside the far corner to put the quarterfinal out of reach.
Xavi's replacement Thiago Alcantara had Barcelona's best chance after that as he shot wide across the face of goal, while Daniel Alves cleared away Milan's most dangerous chance to get back into the game moments before in the 67th.