The Slovakia Bora-Hansgrohe rider was content to bide his time with a measured ride to the top of the six kilometres long Norton Summit, 7km from the end, before attacking thereafter.

He overhauled South Africa''s Mitchelton-Scott rider Daryl Impey to win in 3 hours 21min 07sec at the end of a 128.2km stage from Norwood in which temperatures reached 45 degrees Celsius.

Luis León Sánchez of Spain and Astana Pro Team crossed third as all the leading riders were awarded the same time in an event which doubles as an International Cycling Union World Tour leg.

Sagan now has a two second lead over Impey in the overall standings.

His Bora-Hansgrohe team-mate Jay McCarthy sits third in his home race, a further seven seconds back, after a fifth place finish today.

Sagan conceded that the heat today was among the toughest conditions he had experienced before predicting that tomorrow's 151.5km stage to Willunga Hill would be key.

"We did a Tour of California five years, a stage in Palm Springs, and two guys from the peloton just fell down from the bicycle, so that was really bad," he said.

"For sure tomorrow is a much harder stage than today.

"Tomorrow is going to be a different story."

Australia's Caleb Ewan began the day in the overall lead but dropped out of contention on the first climb today and was not involved in the final sprint.