Downs wasted no time getting in front, with Gaston Sirino getting his first goal for the club as his shot after 70 seconds was fumbled by United goalkeeper Richard Ofori before rolling across the line.
Maritzburg had to wait until the 11th minute to get their first shot on goal, but it was rather wasted as Andrea Fileccia fired over the top.
Sundowns, who have conceded only six goals since their January defeat to Orlando Pirates, were struggling to get a firm grip on proceedings.
The Team of Choice then won a free-kick on the edge of the area but captain Bevan Fransman but was unable to find the target on 25 minutes.
They continued probing for the equaliser and Fileccia looked to charge down on Onyango, but the keeper was off his line in a flash to smother the danger.
Themba Zwane’s defence-splitting pass to find Sibusiso Vilakazi nearly made them pay, but Ofori managed to save from close range on 41 minutes.
It was soon level-pegging as Lebohang Maboe got on the edge of the box and beautifully blasted past Onyango after 43 minutes.