The Brumbies' position at the top of the Australian Super Rugby conference in under threat after going down 13-8 to the Rebels in difficult conditions on Saturday.
In an ugly game played in driving rain, the Rebels played the smarter game as they led from start to finish, avenging their home loss to the Brumbies back in February and claiming their first-ever win in Canberra.
The home sided lack intensity in the opening period and their endless, aimless kicking played in the hands of a hungry Rebels side that had come to the Aussie capital looking to build on their win over the Reds a fortnight ago.
Dominated at scrum time for the first hour, the Brumbies pack failed to match the quality performances they produced for most of the season. Missing same big names from their backline, the Brumbies attack never really fired.
Rebels half-backs Jack Debreczeni and Nic Stirzaker stood out as they engineered a win that revitalises their season.
The Rebels were good value for their 13-5 lead at the interval thanks to a try from wing Sefanaia Naivalu and a couple of penalties from full-back Mike Harris.
The Brumbies were still in the contest thanks to a try on the stroke of half time from centre Henry Speight, but they were being outplayed across the field.
The second half produced just one penalty that saved a losing bonus point for the Brumbies.
Harris opened the scoring with an early penalty before the Rebels bagged the first try on 11 minutes. Stirzaker broke from the back of a scrum on halfway before finding speedster Naivalu, who sold Lausii Taliauli a dummy and raced home.
Harris added the conversion and a further penalty to put the visitors 13-0 up on the half-hour mark.
The Brumbies were rewarded for a patient build up just before the break as Speight crossed at the end of a huge overlap after 15 phases.
Christian Lealiifano's conversion attempt sailed wide, leaving the hosts with a lot of work to do as the teams headed for the warmth of the changing rooms.
The crowd had to wait until the 63rd minute for the scores to change again as Lealiifano landed a penalty to cut the gap to five points but that was she wrote as the Rebels defence stood firm.