Impey sits on the same overall time as Porte, but leads on a countback after having claimed higher stage finishes earlier in the Tour. As a result, the final day's racing tomorrow should be tightly contested, after Porte shone once again today.
The defending Tour Down Under champion, Porte won for the fifth straight year on the summit finish of Willunga Hill, attacking with 1.5 kilometres to go and holding off all challengers to easily take line honours.
Peter Sagan, who came into the day as race leader, was dropped with 1.8 kilometres to go, and Porte took his opportunity to strike, accelerating on the steep final climb.
Bennett tried to match Porte's initial kick, but couldn't manoeuvre past Bahrain-Merida rider Ion Izagirre. As Porte stepped up the pressure, only fellow Australian Jay McCarthy could match him, but Porte found a second wind to coast away to take the stage – but not the overall lead, as Impey stormed home eight seconds behind.
Bennett rolled home shortly after, and looks set to record the best finish by a New Zealander at the Tour since 2010, when Greg Henderson finished in third.
The flatter final stage takes place tomorrow, with Impey and Porte set to battle in a dramatic finish to a punishing Tour.