Ivica Olic hit an injury-time winner to give Bayern Munich the advantage over a poor Manchester United team, and set up a brilliant second leg against the Red Devils.
United took the lead after only 66 seconds when Wayne Rooney volleyed in six yards out from Nani’s free-kick.
But Franck Ribery’s deflected free-kick set up a dramatic finale, rounded off by Olic’s cool run and finish after a defensive error by Patrice Evra.
United’s misery was made complete when Rooney limped off with an ankle injury. The England striker appeared in severe pain after turning over on his right leg in the build-up to Bayern’s winner.
United will hope the injury is not too serious, as they face a crucial Premier League match against Chelsea on Saturday as well as the return match with Bayern next week.
The defeat marked their first in 16 unbeaten European away matches and leaves them with plenty to do, particularly should Rooney be missing in a week’s time and if Bayern are boosted by the return of winger Arjen Robben after a calf problem.