The Germans sat rock bottom of the group before kick-off but resisted United for half an hour until Ruairi Croskey's cool finish put the Reds ahead. Oliver McBurney doubled the lead, stroking home left-footed after cutting inside his marker.
The Reds then conceded their first goal of the tournament in bizarre fashion, a long ball forward looked to be running through to Kieran O'Hara, only for the goalkeeper to unluckily lose his footing on the sodden turf and allow Steffan Hunker a tap-in.
United responded almost immediately, Andreas Pereira firing home from 20 yards to quash any hopes of a comeback. The Belgian struck again to give the hardy fans something to cheer, this time finding the net from nearly 30 yards out as he continues to stamp his class on the competition.
Paul McGuinness' side will face Dublin side Cherry Orchard in the last four of the competition on Thursday.
United: O'Hara; Reid (Dick 53), Tuanzebe (Scott 53), Borthwick-Jackson, Thompson; Riley (McTominay 44), Willock, A.Pereira, Mitchell (Rathbone 58); Croskery, McBurnie (Rashford 44).