A clever strike by Sone Aluko gave Aberdeen victory in an entertaining and ill-tempered clash at Pittodrie.
The on-loan Birmingham player put the Dons in front on the stroke of half-time when he collected Derek Young's pass and slotted home.
Jeffrey De Visscher struck the bar for the Dons in the second-half as they looked to seal the points.
United had Barry Robson and Lee Wilkie sent off before Lee Miller's late strike sealed the points.
Aberdeen made four changes to the team that defeated Inverness in the quarter-finals of the CIS Insurance Cup.
Jackie McNamara and Miller dropped to the bench, with Karim Touzani and Maguire given starting places.
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Andrew Considine replaced Zander Diamond, while De Visscher came in for Jamie Smith, who is suffering from a hamstring injury.
Dundee United named the same side which beat Hamilton 3-1 in midweek at Tannadice.
The Dons went close to breaking the deadlock in the fourth minute when De Visscher's 20-yard shot was deflected wide by Jordan Robertson.
United responded with Lee Wilkie drawing a save from Jamie Langfield after getting a foot to Willo Flood's corner.
On-loan Birmingham player Aluko looked lively for the Dons and he fired the hosts in front on the stroke of half-time.
Aluko gathered a pass from Derek Young before ghosting past Darren Dods and slotting the ball home.
United almost equalised in first-half stoppage-time, but Robertson dragged his shot wide from 10 yards.
After the break Aberdeen continued to look dangerous and De Visscher tested Szamotulski with a header after getting on the end of Aluko's floated cross.
In the 63rd minute, De Visscher's rasping drive cannoned off the crossbar.
Aluko was alert to the rebound, but rather than forcing the ball into Szamotulski's net, the midfielder only succeeded in suffering an injury which saw him eventually replaced by Miller.
United were reduced to 10 men when Robson was given his marching orders for a late challenge on Macguire - sparking a melee involving players from both sides.
Wilkie was then sent off for dissent before Miller's 25-yard effort sealed the points for the Dons in a pulsating encounter.
Aberdeen: Langfield, Hart, Considine, Mair, Byrne, De Visscher, Nicholson, Touzani, Young, Aluko, Maguire. Subs: Kelly, McNamara, Miller, Smith, Smith,McVitie, Paton.
Booked: Touzani, Considine, Young.
Goals: Aluko 45, Miller 90.
Dundee Utd: Szamotulski, Dillon, Wilkie, Dods, Kalvenes, Flood, Gomis, Robson, Hunt, Bauben, Robertson. Subs: McLean, Kerr, Daly, Robb, Kenneth, Duff, Russell.
Sent Off: Robson (78), Wilkie (89).
Booked: Wilkie.
Att: 11,964
Ref: K Clark
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