A superb second-half fight-back from The Dons saw the Tangerines collapse like a pack of cards and the visitors gain some revenge for last year's heavy cup defeat at Tannadice Park.
Going in 2-0 at half-time when it should have been three or four, Jimmy Calderwood fired his team up and sent them out on an attacking mission and they were rewarded against Gordon Chisholm's side which seemed once again to fall into its habitual shell.
It had started so well for the Tangerines as they had grabbed a 26th minute lead. Collin Samuel took the ball on the right flank, teased Richard Foster in and smacked the ball from the by-line for David Fernandez to nip in front of James Langfield and nod the ball home on the goal line.
United fans were ecstatic when, two minutes later, they were gifted a second. Chris Clark seemed to take an eternity on the ball close to the half way line, and he was easily robbed by the impressive Samuel who pushed the ball far forward for Fernandez to get a clear run on Langfield before slotting the ball past the advancing keeper.
Aberdeen were shambolic and dispirited. The Spaniard Fernandez almost carved out his first half hat- trick with a fantastic 43rd minute free-kick, but his shot was matched by Langfield's wonderful save.
With United looking comfortable, the Dons were given a 63rd minute lifeline with a gift from Derek Stillie. With no one near him, he inexplicably dropped a simple cross for ex-Tangerine man Stevie Crawford to accept a late Christmas present and prod the ball in from two yards.
Throughout the season United have been renowned for panicking, and there was no surprise that the North East side drew level in the 69th minute when Crawford grabbed his second, sliding home from 10 yards after excellent work by Clark.
Amazingly the Dons won the game in the 80th minute when a superb three-man move ended with Nicholson hammering home from 12 yards after good interplay between Crawford and Foster.