All but forgotten as a European contender after last season's Calciopoli scandal left it reeling, Milan was far the better team over the two legs of its 5-3 aggregate victory over United, which had been the toast of Europe for its 7-1 demolition of Italian club Roma in the quarterfinals.
Milan moved into the final against Liverpool on May 23 in Athens, which will be a rematch of the 2005 final in which Milan blew a 3-0 halftime lead and lost to Liverpool on penalty kicks.
United had no match for Brazilian Kaka and veteran Dutchman Clarence Seedorf, who both scored in the first half. A second-half goal by substitute Alberto Gilardino completed the rout.
While United has been battling all spring on three fronts, Milan has been able to concentrate on the Champions League and it looked the fresher team. The Rossoneri pressed United from the early going and forced the English club into costly defensive mistakes that produced two Milan goals.
"The first half was the best performance since when I took charge of Milan, both from a defensive and an offensive point of view," said Milan coach Carlo Ancelotti. "We knew we were capable of playing a great match and that's what happened. The way we started, though, was most important. It helped us come through a match that was very delicate and difficult."
The second game resembled the first leg at Old Trafford eight days earlier when Milan came out blazing and scored twice in the first half. But this time, United got back in the game.
United manager Sir Alex Ferguson acknowledged Milan's "fantastic performance" but lamented his players' lack of composure early in the game.
"I expected more from my team," he said. "We'd done so well to get this far, but tonight we never came out of the blocks. In matches like this you have to come through periods when you're under pressure, keep possession until the crowd calm down a bit. I think if we had coped better at the start of the game, and come through the first 25 minutes without cutting our throats it would have been OK. You can't afford to give away goals, and we gave away two soft ones tonight."
"AC Milan were better prepared physically than us. Over the last few weeks, they've been resting players at the right moments, while we've had a tough week in England," he said. "We've been using the same players for weeks without respite. We also had to come the extra mile to beat Everton on Saturday. That's what made the difference".
May 2 in Milan
AC Milan (Italy) 3 Man. United (England) 0. Goals: Kaka 11, Clarence Seedorf 30, Alberto Gilardino 78.
AC Milan -- Dida, Oddo, Nesta, Kaladze, Jankulovski, Gattuso (Cafu, 84), Pirlo, Ambrosini, Seedorf, Kaka (Favalli, 86), Inzaghi (Gilardino 66).
Man. United -- van der Sar, O'Shea (Saha 77), Brown, Vidic, Heinze, Fletcher, Carrick, Scholes, Cristiano Ronaldo, Rooney, Giggs.
Referee: Frank De Bleeckere (Belgium).
(AC Milan win 5-3 on aggregate.)