Daryl Impey and Richie Porte will fight out the most dramatic finish in the Tour Down Under's 20-year history.
Porte won the pivotal Willunga stage for the fifth year in a row on Saturday, beating Impey by eight seconds.
That leaves Porte and Impey tied for first place overall with one stage left. Impey leads on a countback of previous stage results.
Porte was aiming to become the first champion in the Tour's 20-year history to defend his title.
The BMC team leader unleashed his trademark attack on the climb to the summit finish at Willunga in Saturday's 151km fifth stage.
Porte started the day 14 seconds behind race leader Peter Sagan on the overall standings, but Sagan wilted early on the climb.
Sagan's teammate Jay McCarthy (BORA-hansgrohe) initially stayed with Porte when he attacked with 1.5km left.
But McCarthy had no answer when Porte attacked again, just before 1km to go.
McCarthy was third overall at nine seconds heading into Willunga and was considered Porte's biggest rival.
But Impey, a wily veteran who rides for Australian team Mitchelton-Scott, sprung a big surprise. He managed to break clear as well near the finish, finishing a clear second and restricted his time loss to Porte.
Impey had started the stage second overall, two seconds behind Sagan, but his Tour history indicated that he was unlikely to stay with the leaders on the Willunga climb.
Along with beating Impey by eight seconds, Porte picked up four bonus seconds for winning the stage, putting them level on overall time.
Impey has placed better in earlier Tour stages this year, meaning he has the race lead on countback.
It means Porte's BMC and Impey's Mitchelton-Scott will have a titanic battle at the intermediate sprint and king of the mountain checkpoints during Sunday's stage, as well as the finish.