Impey (Mitchelton-Scott) picked up five bonus seconds in the intermediate sprints on Saturday's fifth stage to wipe off previous leader Richie Porte's advantage.

Giacomo Nizzolo (NTT Pro Cycling) crossed the finish line first at Victor Harbor ahead fellow Italian Simone Consonni (Cofidis) and stage-one winner Sam Bennett (Deceuninck-Quickstep).

Australian sprint star Caleb Ewan (Lotto-Soudal) was eighth, but all eyes were on the general classification battle between Impey and Porte (Trek-Segafredo).

The South African ace will take a two-second lead into Porte's pet stage on Sunday, when the Tasmanian will start hot favourite to win the Willunga climb for a seventh-consecutive year.

"The whole week, we've been trying to get into the lead here and to put myself into a position tomorrow where it's achievable to beat Richie," Impey said.

"Two seconds is going to be really tight and close, but we've given ourselves the best chance possible.

"The boys have ridden incredibly hard all week and done an amazing job.

"Win, lose or draw, at the end of the race tomorrow, we know we've given it everything."

For the second-successive stage, Porte had world champion teammate Mads Pedersen to thank for ensuring Impey didn't take more bonus seconds and build a bigger overall lead.

"Mads did a good job to take some seconds, so it's not a massive deficit," Porte said.

"Mitchelton were good today, they exploded it there in the final and it's going to be a hard race tomorrow.

"I think if we can win the stage, we will win the race, but we're going to have to have guys up there.

"I was a little isolated today so, hopefully tomorrow, the boys are on song."

Nizzolo's stage win was his first at the Tour Down Under.

"At the team presentation, I picked this stage," he said.

"I thought it could suit me quite well."

Australian Associated Press