Wits have now gone without a win in 12 matches after Mphela's headed goal, a 57th minute red card for captain Tefu Mashamaite and a disallowed first half effort by Kadi sealed their fate.
Sundowns struggled in the first half with little cohesiveness despite edging the position over Wits who were more direct and quick with their attacking.
In truth, there was little excitement in the opening 45 minutes as the home side seemed content to sit back and allow their visitors to come at them.
The Clever Boys did have the ball in Downs' net in the 19th minute but the goal was disallowed when Mark Haskins beat the offside trap, rounded keeper Brian Baloyi but instead of going for goal, Haskins then passed to Calvin Kadi who netted into an empty net but was ruled offside by referee Victor Hlungwani on the insistence of his assistant Stephen Moshotle. The call looked marginal.
The Students kept up the pressure throughout the opening half but couldn't get the goal to break the a 0-0 scoreline – which remained until half-time.
In the second half, Katlego Mphela failed to capitalise on a goalkeeping mistake from Wits teenager Darren Keet in the 52nd minute.
The tide of the match turned for goal in the 57th minute though. The Students' skipper and defensive lynchpin Mashamaite was shown a straight red card by referee Hlungwani after the defender pulled down Mphela who was racing for goal and was moving into a goal-scoring position.
Two minutes later Keet produced a tremendous save to deny Downs hard working midfielder anchorman Matthew Pattison a goal when he tipped Pattison's goal bound shot over his crossbar.
From that moment on the pressure from Sundowns on Wits became relentless, despite Wits' coach Roger de Sa attempting to reshape his team with tactical substitutions.
The pressure finally told six minutes from time when Mphela brought his goal tally in the league to 13. He headed home Pelembe's 84th minute corner to give Downs three valuable points and keep them in the title race.