Real Madrid had the first chance of the half after Michael Essien dispossed Xavi to start a counter attack break. Cristiano Ronaldo found Karim Benzema at the top of the 18-yard-box with a celver backheel and Benzema worked laterally before placing a shot that curled just over the bar.
Minutes later Barcelona finally struck.
Lionel Messi took the ball right off Xabi Alonso's foot and tapped it forward to Cesc Fàbregas who was sitting onside in a ton of space behind the majority of the Madrid back line. Fabregas was in alone and took his time, drawing Diego López off his line before beating him with a shot towards the far post as López committed to his right.
Madrid really have been the better side overall through the first half and at the start of th second half, so giving up a goal like that will sting. It also won't help that this is a home-and-home tie and away goals count as a tiebreaker after 180 minutes. An away goal for Barca who return home for the second leg could be huge.