Cabous Eloff's late try helped the Rebels edge the Force and reach the Super Rugby AU finals.
The ruthless Brumbies scored five tries to see off Western Force and ensure they will be at home in the final on September 19.
James O'Connor scored a try and kicked 17 points as the Reds achieved a record win over Western Force on Friday.
The Waratahs secured back-to-back Super Rugby wins for the first time in almost a year and a half on the Gold Coast.
After the scores were deadlocked at 20-20 after 80 minutes, Isi Naisarani's try sealed victory for the Rebels against Western Force.
The Brumbies are the only team to have won every Super AU game after they were too strong for the Western Force.
Western Force made a flying start in their first Super Rugby AU game, but the Waratahs fought back to secure the victory in Sydney.
After a three-month suspension due to coronavirus, Super Rugby will return in Australia from July 3, albeit in a revamped format.
An Australia-based Super Rugby competition starting later this year will include the Force.
Rugby Australia has invited Western Force to join the Rebels, Waratahs, Reds and Brumbies in a new competition.