Pirates made an energetic start and after gaining possession, Innocent Maela picked up the pieces before making an over-lapping run to shot wide of goal.
Bucs were on the charge timeously, leaving little space to United as Augustine Mulenga took sight of goal which sailed wide.
The Zambian ace later on amended his earlier discrepancy, poking the ball past Ronwen Williams from close proximity to open parity.
United drew level with a pile-driver from Dean Furman just outside the box on 33 minutes after some fine build-up play involving Evans Rusike.
Good work set-up Evans Rusike for a low-cut finish past a scrambling Mpontshane to put SuperSport ahead. It lasted all but a mere 10 minutes as Bucs pulled level.
Defender Gladwin Shitolo capitalised on slack defending to deflect home Xolani Mlambo's shot inside the box on 60 minutes.
Neither side were able to break through thereafter, despite the offensive reinforcements brought from the bench.
As such, the tie went into 30 minutes of extra time. With penalties approaching, Pirates got the chance to go ahead after being awarded a penalty.
Mulenga though, known for his powerful drive, was unable to beat Williams who spared his blushes in front of the boisterous home base, forcing the game to be settled on a shootout.
A few misses from Bucs, allowed James Keene the opportunity to send United in the semis and he duly obliged with the final spot kick for victory.