Now on 58 competition points heading into the final round, they would have to lose to the lowly Blues at home next weekend and Queensland (53) would need a bonus-point victory over NSW to unseat the Canberra-based side from top spot in the Australian Conference.
Coach Jake White would be concerned at a second shaky performance from his side following the June Test break, but the result, breaking a 10-year drought in Sydney, was all that mattered against a Waratahs side with nothing to lose.
NSW dominated in most areas and had all the running in the first half, but a try to winger Henry Speight and sideline conversion from Zack Holmes in the 62nd minute got the Brumbies to a lead which they never gave up.
The Brumbies, under-rated all season, looked off the pace at the breakdown but two penalty kicks from Holmes and a long-range effort from Jesse Mogg gave the visitors a 9-5 lead at halftime.
However, the Waratahs saw immediate benefit from a new backline combination and an attacking mindset, with Tatafu Polota-Nau crashing over in the first half and Berrick Barnes forcing his way over in the 57th minute to give NSW a 15-9 lead.
But in the end it was to no avail, with the Brumbies showing composure under pressure before Holmes threw a looping long ball for Speight to score the vital try.
NSW reserve halfback Brendan McKibbin had a shot from 49 metres out which would have brought them within a point heading into the final 10 minutes, but it sailed wide.
The Brumbies missed out on the bonus point which would have absolutely guaranteed them a home final.
If teams are locked on points at the end of the regular season, it comes back to most wins and at the moment the Brumbies and Reds are locked with 10 wins each.
Queensland also remain a chance to still force a wildcard finals spot, as would the Brumbies if they lose to the Blues and the Reds take maximum points over the Waratahs.
White said he noticed his players were nervous during the day, but believes they will be better for the experience.
"We didn't play our best rugby. We looked like we got the jitters during the day and it looked like we got a bit scared, and it's natural playing against 11 Wallabies and at a ground (city) where we haven't won in a while," said White.
"In nine games now in a row no team has scored in the last 20 minutes against us so all credit to our conditioning staff.
"As much as you don't want them to look at the table ... it's the most natural thing in the world.
"We've managed to keep the destiny in our own hands by keeping in front of everyone else."
Waratahs coach Michael Foley had doubts about Holmes's pass to Speight but conceded his men were once again their own worst enemy as they slumped to their 11th loss.
Captain Benn Robinson, playing his 100th game for NSW, said the second half summed up their season.
"First half we played some really solid footy, the boys stuck to strategy and put some pressure on the Brums," he said.
"In the second half we had some complacent actions which really hurt us and let them back in the game. It's always disappointing to come this close. We give up too many penalties and let teams back in the game."
There's a concern over Waratahs No.8 Wycliff Palu, who damaged his shoulder and was replaced in the first half.
