The best player in the world took just five minutes to offer a riposte in the return.
Messi took possession on the edge of Milan's box and, despite being crowded by white shirts, shuffled the ball across to his left foot and found the top corner with some of the fleetest footwork you're likely to ever see.
The goal changed the complexion of the tie. Milan started the night with a 2-0 advantage, but Messi's early strike served to invigorate his teammates and lift the crowd at Camp Nou.
Milan goalkeeper Christian Abbiati was helpless as Messi's exquisite shot found the top corner at a speed that belied its lack of back-lift.
Messi wasn't done yet. In the 40th minute he brought Barca level at 2-2 on aggregate with a clinical low finish through a defender's legs.
The two teams went into the break all square—and the Barcelona comeback was fueled entirely by the genius of Messi.