The Uruguayan forward scored at Anfield's Kop end in extra time to put Atletico Madrid through on away goals after an aggregate score of 2-2. Forlan also netted in the first leg in Spain last week, giving him a career tally of four strikes against the Merseysiders - following his famous double for United in December 2002.
Rafael Benitez's men must now pick themselves up for Sunday's penultimate match against Chelsea but their ability to beat the league leaders - and consequently hand the title-race advantage to United - may be hampered by playing for an intense two hours, not to mention the heartbreak of losing a tie they were all set to win through two goals on the night by Alberto Aquilani (44 minutes) and Yossi Benayoun (95).
Atletico will meet Fulham in next month's final after Roy Hodgson's side came back from 0-1 down to beat Hamburg 2-1 in their semi-final second leg at Craven Cottage.
Video: Relive United's 2-1 win at Liverpool in 02/03, featuring Forlan, on MUTV Online.