The hosts took the lead in the fifth minute, when Papi Mngomezulu turned home Thabiso Hlalele’s pin-point pass with a deft back-heel.
They netted a second in the final minute of the game when Siza Dlamini converted an 89th minute penalty.
Sundowns were aware that SuperSport United were losing their game to Bidvest Wits, but in one of their most crucial matches of the season they were unable to lift themselves and break down a resilient defensive Cosmos performance.
Cosmos stunned Sundowns when, with their first real foray up field, they took the lead. It came after some good work by Hlalele on the right flank saw the Cosmos midfielder dazzle his way past James Mayinga to set himself free.
With all the time in the world he managed to turn in an accurate pass to Mngomezulu inside the Sundowns six yard area, who flicked the ball with his heel past a bemused Calvin Marlin and a static Sundowns defence to give Cosmos the lead.
The home team was playing like they wanted to avoid relegation and were able to keep possession for lengthy periods. One of these spells on 14 minutes saw Norman Smith able to set free Diarra on the left edge of the Sundowns 18 yard area.
With only Marlin to beat he panicked and shot across goal, missing the target when he should have instead picked his spot and had Cosmos two goals to the good ahead.
Three minutes after that and Matthew Booth set free Katlego Mphela with a superb long ball played along the ground towards the striker, who picked up the ball and stormed down the right before cutting it across to Papi Zothwane.
The lanky midfielder, stationed inside the Cosmos danger zone, was able to walk with the ball towards goal unmarked before letting fly at goal, only for Avril Phali to come up with an amazing one hand save to block the ball and keep his team in the lead.
Ezenkosi continued to search for an insurance goal and when the opportunities did fall for them they failed to capitalise. Smith was the guilty party in the 31st minute when put through on goal by Gaberone: in a one on one situation with Marlin he shot into the Sundowns side netting.
Franklin Cale forced Phali into making a save when he unleashed a powerful shot at goal from 20 yards out from the right edge of the Cosmos 18 yards area after being set up by Khenyeza. The first half ended with Cosmos leading 1-0.
Soon after the restart Cosmos failed to \ deal with a troublesome ball from the right taken by Cale. Although the ball bounced over their defence they let off as Zothwane, who was unmarked at the back post, could not make contact with the ball and head home.
Sundowns were getting desperate for the equaliser and were throwing men up front in search of the goal which would keep them in the title hunt. Cosmos managed to get enough bodies behind the ball to thwart Sundowns’ wave after wave of attacks.
They hit Sundowns with a swift counterattack in the 58th minute. As Gaberone collected the ball in his own half he raced up the field and from around 20 yards from goal he blasted the shot, Marlin judged the flight of the ball and half crouched got down to make the save.
One would have betted a month’s wages on Mphela drawing Sundowns level in the 65th minute when he found himself free and unmarked on the end of Elias Pelembe’s beautifully floated ball. Somehow, with no one in front of him and with a free header at goal, Mphela headed the ball over the Cosmos upright.
In the final minute of regulation time Sundowns finally saw their hopes of landing a sixth PSL title evaporate when James Mayinga brought down Norman Smith in the penalty area to hand Cosmos a spot-kick.
Dlamini stepped up and buried the ball past Marlin to destroy any hopes Sundowns had of taking the title race to the final day of the season.