[Source: Rugby.com.au]
The Brumbies had a narrow 10-7 lead heading into the break before the brilliance of Len Ikitau sparked them into life.
It gave them a 22-7 lead before the Waratahs launched the comeback in the pouring rain.
They scored twice in 15 minutes to set up the grandstand finish as they fought to keep their season alive.
However, the Brumbies held it together and delivered the winning blow when Sefo Kautai slid over the line with three minutes to go.
While the Brumbies are assured of finals football, Darren Coleman’s Waratahs side will need to win their remaining three matches to scrape into the playoffs.
The Brumbies enjoyed all the early running and earned a 7-0 head start with a ninth-minute try to No.8 Charlie Cale.
Already without six front-rowers, the Waratahs suffered another hammer blow in the shadows of halftime when prop Harry Johnson-Holmes limped off with a leg injury.
But, refusing to go away, the Waratahs struck back shortly after through inspirational halfback and captain Jake Gordon.
For all their possession and territory, only a late penalty goal from Ryan Lonergan earned the Brumbies a three-point halftime buffer.
Tries to Wallabies pair Len Ikitau and 2023 John Eales medallist Rob Valetini looked to have given the Brumbies an unassailable 22-7 lead.
But, despite the deplorable conditions, the Tahs hit back with tries to centre Joey Walton and five-eighth Tane Edmed to narrow the deficit to a single point.
Kautai, though, had the final say as the Brumbies notched up a 13th straight victory over the Waratahs in what has become a one-sided arch-rivalry.
