The Brumbies battled to a 15-19 victory over the Waratahs in Sydney to hold on to their top spot in the Australian conference.
The Brumbies led 5-9 at halftime and scored their only try through Henry Speight, while Zack Holmes and Jesse Mogg kicked two penalties each.
Berrick Barnes and Tatafu Polota-Nau crossed for Waratahs tries in the team's 11th loss in 15 matches this year.
The Waratahs play defending champion the Reds in the final round next weekend, with the Brumbies hosting the Blues.
The Reds face an uphill battle to qualify for the six-team playoffs after the Sharks beat the Bulls with a bonus point earlier in the weekend.
The victory means that the Brumbies maintained their five-point lead over the Reds and also secured their first win in Sydney for a decade and their fifth consecutive away victory.
They will seal the conference and their place in this month's top-six playoffs by earning at least one point against the lowly Blues in Canberra next weekend.
The Waratahs hit the front 15-9 with just over 30 minutes left in a grinding Australian derby at the sparsely-populated Olympic stadium, only for the Brumbies to come home the stronger in the closing stages.
Fijian winger Henry Speight scored off Zack Holmes' floating pass, evading Wallabies' backs Adam Ashley-Cooper and Drew Mitchell on the right wing just after the hour.
The Waratahs, with 10 Wallabies in their starting XV, have now lost seven games by four points or less this season and are well out of contention for the playoffs later this month.
The Waratahs scored two tries to the Brumbies' one with young flyhalf Holmes landing two penalties and a conversion and Jesse Mogg two penalties.
"It wasn't exactly the victory we were after, but you've got to give credit to the 'Tahs, they are a quality side across the park and coming up here and winning, it's not something the Brumbies have done since 2002," skipper Ben Mowen said.
"We keep breaking these hoodoos, we keep taking a step forward."
The Waratahs looked threatening at times and scored tries through Wallabies Tatafu Polota-Nau and Berrick Barnes, but made some crucial errors in the final quarter as the Brumbies hit back.
"It wasn't a good performance by us, we're a better team than that," Waratahs skipper Benn Robinson said.
"The Brumbies came out and played a solid game tonight and they deserved to win."
"As a team we'll definitely learn and grow from this. It is disappointing when we lose so closely."
The Waratahs can end the Reds' season if they win in Brisbane next Saturday in their final game of an underachieving campaign.
Final Score Waratahs 15 (5) Brumbies 19 (9)
Tries - T. Polota-Nau, B. Barnes
Pen - B. Barnes
Con - B. Barnes
Tries - H. Speight
Pen - Z. Holmes 2, J.Mogg 2
Con - Z. Holmes
Referee: Keith Brown
Assistant Ref 1:Ian Smith
Assistant Ref 2:Julian Pritchard
TMO : George Ayoub
15. Adam Ashley-Cooper ,14. Lachie Turner ,13. Rob Horne ,12. Berrick Barnes , vice captain,11. Drew Mitchell ,10. Bernard Foley ,9. Grayson Hart ,8. Wycliff Palu ,7. Chris Alcock ,6. Dave Dennis ,5. Kane Douglas ,4. Dean Mumm ,3. Sekope Kepu ,2. Tatafu Polota-Nau ,1. Benn Robinson , captain
Replacements:16. John Ulugia,17. Paddy Ryan ,18. Sitaleki Timani , 19. Lopeti Timani ,20. Brendan McKibbin ,21. Tom Carter ,22. Tom Kingston
1. Ben Alexander, 2. Stephen Moore, 3. Dan Palmer, 4. Scott Fardy, 5. Sam Carter, 6. Peter Kimlin, 7. Michael Hooper, 8. Ben Mowen, 9. Nic White, 10. Zack Holmes, 11. Jesse Mogg, 12. Pat McCabe, 13. Andrew Smith, 14. Henry Speight, 15. Robbie Coleman.
Replacements: 16. Anthony Hegarty, 17. Ruaidhri Murphy, 18. Leon Power, 19. Ita Vaea, 20. Ian Prior, 21. Tevita Kuridrani, 22. Cam Crawford.