The Brumbies punished the Waratahs for their lack of execution, with 37-year-old lock Cadeyrn Neville proving an unlikely hero in Friday night's tense victory at Allianz Stadium.
While Australia's benchmark Brumbies also kept alive their hopes of a crucial top-four finish, another frustrating home defeat for the Waratahs left Dan McKellar's erratic outfit needing a miracle to make the playoffs.
The Tahs must beat the Western Force in Perth next Saturday and pray the sixth-placed Queensland Reds lose their remaining two regular-season games - against Moana Pasifika in Auckland and Fijian Drua in Brisbane - to scrape into the playoffs.
Finishing in the top four shapes as critical under the new finals format, which pits first versus sixth, second against fifth and third versus fourth in the opening week of the playoffs.
The highest-ranked losers in the first three finals - which are all likely to be staged in New Zealand - will advance to the semi-finals.
If the Brumbies beat Moana Pasifika in Canberra and the Crusaders lose to the table-topping Hurricanes in Christchurch, Stephen Larkham's team will snatch fourth.
The Brumbies could conceivably finish third and snare a home final if the second-placed Blues also lose to the second-placed Chiefs next weekend.
For now, though, they can celebrate sealing a finals berth and regaining bragging rights after avoiding losing twice to the Waratahs in a single season for the first time since 2015.
"You've got to be able to win in a variety of ways, in this competition in particular," Lonergan said after the Brumbies overcame an alarming lack of possession and territory to prevail.
NSW were left cursing themselves after dominating but blowing numerous try-scoring chances to somehow trail 14-0 at the break, then fall short in a spirited late-match fightback.
"That game, we lost to ourselves," frustrated winger Max Jorgensen said.
"We hurt ourselves so many times - getting held up, loose ball, loose carries, and poor discipline. You can't win games if you're doing that.
"We should have been up by 20 in the first half. We had all the possession, we were in their half the whole time and we just couldn't get the job done."
Neville was the Waratahs' nemesis.
First he defied age and physics to pull down Jack Bowen just as the Tahs flyhalf seemed certain to score.
Then he crossed for the Brumbies' second try, after scheming scrumhalf Ryan Lonergan put hooker Billy Pollard over for their first against the run of play in the 22nd minute.
Lonergan also looked set to score before his opposite number Teddy Wilson pulled off a desperate try-saver, only for Waratahs flanker Charlie Gamble to be yellow-carded for offside intervention in the ensuing play.
The Brumbies wasted no time doubling their lead through Neville's try a minute later.
The visitors looked home and hosed when prop Allan Alalaatoa celebrated his 150th game with a rare try to take the visitors out to a 21-0 lead.
But late tries after the hour mark to Waratahs prop Isaac Kailea and Jorgensen set up a grandstand finish as the two arch rivals battled desperately to keep their seasons alive.
