A fascinating battle between the two sides which will make an interesting second leg at the Nou Camp.
Barcelona started in their 4-3-3 with Iniesta on the left of the front three and Keita taking up a place in midfield. AC Milan started in a 4-3-1-2 system with Bonera filling in at right back and Boateng supporting Robinho and Ibrahimovic up front.
Milan began on the front foot, pressing Barcelona high up with their first and second line and forcing Guardiola's defenders into making mistakes. Robinho had an early chance on a volley on the left but was unable to get it on target. However Allegri's men had come into the game with good defensive strategies. They pressed high up on Barca's defenders from goal kicks and when they had it deep in their own half. Then when Barcelona worked the ball around in space with Ibrahimovic and Robinho ahead of the ball, Milan retreated to the edge of their area and tried to make it compact in the middle.
Barcelona of course dominated possession as usual with mobility and interchanging in the middle creating space to penetrate with Messi in particular finding gaps between Seedorf and Ambrosini who himself had a good game. Milan's midfield didn't generally put pressure on in this area which did make them vulnerable when Barcelona switched the ball out, enticing Milan's midfield higher, and then attacked the space. However because it was so compact, Barcelona struggled to break through, unable to make their usual wall passes to penetrate behind. Iniesta played on the left when defending and in the initial phases of possession but then swapped with Keita so the Spaniard could move in and give numerical superiority between the lines and work space there.
Apart from when Iniesta took him on 1v1, Bonera at right back was rarely tested because Puyol generally stayed fairly deep on the left to give defensive balance. Barca's width came from Dani Alves moving up high on the right but he didn't have a great game, though Milan generally gave him space, preferring to concentrate on keeping the middle compact. It was noticeable that Boateng dropped deep from the front three to help defend while the other two generally remained high for quick transitions.
Still Barcelona looked threatening with their interplay in the middle with Alexis deserving a penalty after Abbiati fouled him and Messi having a goal disallowed from a quick counter after Milan pressed high up the pitch. Milan themselves generally got numbers up in attack and moved the ball quickly with Boateng often moving to the left in attack, with Robinho inside and Ibrahimovic moving more to the right between Pique and Puyol. Milan got a good chance from this when they again won the ball high up and Ibrahimovic was played in but he had his shot held by Valdes down to his left.
Neither team changed too much at the start of the second period though the Milan midfield was slightly higher up initially. Iniesta was able to cut inside early on and take a shot from just outside the area, a warning to Milan. However the home side continued to deal with the threat well. Alexis was continually making space for Alves on the right by making diagonal runs across Antonini to drag the left back inside but the Brazilian failed to have a big impact on the game.
El Sharaawy came on for Robinho six minutes into the second half and played more to the right than the Brazilian had been with Ibrahimovic moving more to the left. The Swedish striker's hold up play was good and he also looked to move behind Barcelona's defenders though was caught offside a couple of times.
After a while Guardiola made a change, taking off Iniesta for Tello on the left who provided better width and a more direct threat by running 1v1 against the right back. He created space for himself for a shot on goal which hit the side netting not long after coming on and then minutes later pulled the ball back for Messi in space to have a shot from the edge of the area.
Meanwhile Milan looked to attack with direct balls to Ibrahimovic with Emanuelson and El Sharaawy supporting him and looking for a knock on or pass behind to take advantage of the space behind Barca's defenders, who often didn't cover vertically behind the two players in the aerial duel.
Barcelona were unable to fashion clear chances late on, with Milan doing what Leverkusen failed to do in the last round; limiting quick transitions. They did this by getting numerical superiority in the centre on the edge of the area and making it hard to play through them. The closest the away side went late on was through a Xavi shot just outside the area which went wide of the goal after good movement from Barca in the middle to create space.
The game ended goalless and Milan take a 0-0 draw to the Nou Camp, becoming the first team in two and a half years, since Rubin Kazan, to stop Barcelona scoring in a Champions League game. Barcelona however will rightly still be favourites to progress through to the semi-finals.