Playing with a man down for 72-minutes, the Brazilians were always playing catch-up after influential defender Clayton Daniels was sent off in the 18th minute of the match.
The loss in numbers, however, forced Downs to knuckle down and for a brief period they dominated proceedings.
Ultimately, however, United made their numerical advantage count with goals in the 39th and 61st minutes to leave the fancied Sundowns outfit on the ropes for most of the match.
Earlier, in the seventh minute, Sundowns unlocked the SuperSport defence and Nyasha Mushekwi was left one-on-one with Ronwen Williams, but the keeper managed to grab the ball away from the Downs striker’s feet.
A minute later United replied through Sibusiso Zuma, but his effort from point-blank range was blocked by Wayne Sandilands.
Zuma, showing his frustration after missing the straightforward chance, tried his luck from long range in the 11th minute but his shot was well wide of the target.
Sameegh Doutie caused some trouble for the Sundowns defence with a number of probing runs, but his crosses failed to find their mark.
Kermit Erasmus was then fouled just outside the area by Daniels, who was sent off in the 18th minute, to give United a clear advantage.
Tebogo Langerman took a shot from the resultant free kick but his attempt just shaved the crossbar.
Sundowns seemed to click after they had their numbers reduced as they began to threaten the SuperSport goal.
Elias Pelembe took a free kick from near the corner flag in the 25th minute, and Williams had to tip the attempt away from goal.
It left Teko Modise with a free shot at goal, but he wasted the chance with a wild swipe into the crowd.
United, though, began to control the game and were rewarded with the opening goal in the 39th minute.
Doutie whipped in a corner that left Thabo September unmarked at the far post and he obliged by tapping home to make it 1-0 to United.
Modise had another go at goal in the 52nd minute from the left-hand side, from long range, but again his radar was off.
Two minutes short of the hour mark, Elias Pelembe found himself in good position in space from close range on the right, but he was off balance when he tried to shoot and banged his attempt into the stands.
Erasmus put United 2-0 up in the 61st minute with a well taken lob over the keeper.
He first had a shot blocked by Sandilands, but he was presented with the rebound and the 21-year-old produced a clever shot over the head of the Downs keeper to leave the Brazilians with a mountain to climb.
Zuma looked to be in good position to make it 3-0 in the 69th minute, but his unmarked shot from the left-hand side again went well over the crossbar. — Sapa.