The win maintained UnitedÃ¢â‚¬â„¢s enviable away record in the competition, who are now unbeaten in sixteen games on the road. The return leg will be played at Old Trafford in three weeks.
The intimidating atmosphere of the Stadio Giuseppe Meazza appeared to rattle the visiting side in the first half. Indeed they were sloppy, complacent and produced a catalogue of errors as AC Milan conjured up a hat-full of chances.
Ronaldinho scored within three minutes of the game commencing. United conceded a cheap free kick which Beckham duly took. Patrice EvraÃ¢â‚¬â„¢s sub-standard clearance fell to Ronaldinho and the Brazilian duly fired the ball past Van der Sar, albeit thanks to a fortunate deflection off Carrick.
United looked limp in midfield, surrendering possession to the Rossoneri and failing to find any passing rhythm. Ferdinand led a nervy rearguard which came under heavy pressure following RonaldinhoÃ¢â‚¬â„¢s early finish.
The Brazilian brought back evocative memories showing regular flashes of brilliance that had previously been synonymous with the former Barcelona ace. The ingenious attacking midfielder almost had the ball in the net again but was denied by the outstretched arm of UnitedÃ¢â‚¬â„¢s keeper.
The wave of Milanese attacks kept coming.
A lengthy run out of defence by Thiago Silva saw him stride through the midfield and unleash Antonini. The left back outwitted the young Rafeal before snapping his shot the wrong side of Van der SarÃ¢â‚¬â„¢s post.
Milan drove forward with dynamism. Scholes carelessly presented possession to Ronaldinho who fearlessly ran at Ferdinand, attempting to bamboozle the England captain. But the centre back did just enough to bring the Brazilian to a halt.
Jonny Evans endured a torrid first half but did produce a crucial challenge on Pato at the half hour mark. It brought to an end a dangerous-looking run for the striker after Carrick had cheaply given away possession.
Former Arsenal and Spurs target Klaas-Jan Huntelaar spurned a glorious opportunity to put the home side firmly in control of the tie. Evans played a woeful ball out of defence which fell to the giant Italian, Ambrosini.
The midfielder slid his Dutch team-mate through on goal. However Huntelaar blasted his shot wide much to the despair of the tifosi inside the San Siro.
Moments later Milan were punished for their wayward shooting.
A snappy one-two exchange between Fletcher and Park saw the Scottish international dart to the by-line. He delivered a low ball across DidaÃ¢â‚¬â„¢s box which kindly struck the left shin of Scholes and bobbled past the despairing palms of the Milan goalkeeper.
The first half belonged to Ronaldinho and he had the final chance of the half. A delightful turn saw him prance past Scholes and Carrick. He lined up a bobbling shot which proved awkward for Van Der Sar, who parried wide.
The Brazilian was quite simply majestic and if he can produce such performances in the coming months in Italy then he must make the trip to South Africa.
The second half, however, belonged to an Englishman.
Wayne Rooney led United from the front as he hounded the Italian defenders in search of a second. He picked up the ball at the half-way line and sprinted towards DidaÃ¢â‚¬â„¢s goal with his shot deflected wide for a corner.
He sent Nesta and company a second warning when he effortlessly burst past Silva and dragged a low shot wide of the goal. He was getting closer and crucially was becoming a key figurehead in the match.
Antonio Valencia replaced Nani, who had endured a torrid time, and had an almost immediate impact. He dribbled to the right by-line and delivered a perfect ball for his striker. Rooney duly nodded past the helpless Dida.
Less than ten minutes later United added a third. A disguised cross from Darren Fletcher found Rooney completely unattended within the Milan box. The United number 10 escaped the attentions of the motionless Nesta and Silva to glance the ball into the net once more.
The atmosphere drained from the stadium as a stunned San Siro watched Milan crumble in emphatic fashion.
The introduction of Clarence Seedorf late on provided some hope to the Rossoneri supporters. Ronaldinho produced a quick step-over before poking a pass to Seedorf. The Dutch wizard sublimely flicked the ball into the United net.
It left Milan with some hope of resurrecting the tie at Old Trafford.
AC Milan (4-3-3): Dida; Bonera, Nesta, Silva, Antonini (Favalli); Ambrosini, Pirlo, Beckham (Seedorf); Ronaldinho, Huntelaar (Inzaghi), Pato.
Manchester United (4-1-4-1): Van der Sar; Rafael, Ferdinand, Evans, Evra; Carrick; Nani (Valencia), Scholes, Fletcher, Park; Rooney.
Man of the Match: A virtuoso performance by RonaldinhoÃ‚Â merits the title of man of the match. However Wayne Rooney scored the telling brace which keeps United in the lead as the two teams prepare for a crunch clash at Old Trafford. Many will see it as his best display in the Champions League as he finally produces a world class performance away from the safety of Old Trafford.
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