Toni Pulu was red-carded against his former team as defending champion the Brumbies beat Western Force 21-9 to set up a Super Rugby AU final against the Reds.
The semi-final at GIO Stadium on Saturday was well poised with the holders 7-3 up when wing Pulu was dismissed for a dangerous tackle on Len Ikitau.
Pulu could have no complaints, having caught Ikitau in the face with his shoulder, and his early exit in the closing stages of the first half proved to be costly.
Domingo Miotti's penalty put the Force in front, but Tom Wright crossed in the corner soon after at the other end for a try converted by Noah Lolesio.
Tom Banks then finished on the stroke of half-time to rub salt into the Force's wounds after Pulu's departure.
Five-eighth Miotti reduced the deficit to 12-6 when he was on target from the tee early in the second half, but Lolesio booted the Brumbies back into a nine-point lead.
Wright had what he thought was his second try ruled out after a replay showed his foot was in touch in-goal as he touched down, but two penalties from Ryan Lonergan gave the Brumbies breathing space.
Dan McKellar's side will face the Reds in the final at Suncorp Stadium next Saturday.