What happened next shows why he is hailed as Australian road cycling's new sprint king.

Ewan emerged from the slipstream of Peter Sagan and tore past the superstar on Wednesday for the best of his seven Tour Down Under stage wins.

The stunning stage-two upset also gave the 23-year-old sprint ace the overall lead.

It is a major confidence boost at the start of a crucial season, in which Ewan will make his much-anticipated Tour de France debut.

"It's probably more exciting, winning on a stage where you're a little bit unsure," Ewan said.

He admitted his confidence was low after finishing third in Sunday's Classic street race and runner-up in stage one.

The uphill finish meant Sagan was one of the stage favourites, whereas Ewan was an outsider.

At the base of the final climb, Ewan was still unsure whether he could contend.

Mitchelton-Scott's Daryl Impey was also in the final selection and is a rider more suited to the Stirling finish.

"It was a tricky one because, obviously, I didn't know if I was going to get up there or not," Ewan said.

"I didn't want to make the call from the bottom ... I didn't know how I was going to feel.

"I saw Daryl sitting up there pretty nicely as well, so I followed him and told him to go (lead out Ewan's sprint) with about 250-300 to go."

Ewan had his fists in the air as he crossed the line and joked with Impey soon after.

"He said 'you've got to let me win one'," Ewan said.

"I didn't actually know he got second."

Mitchelton-Scott team director Matthew White revealed Ewan had been sick since this month's road nationals, where the sprinter won his third-straight criterium championship.

"This sprint doesn't normally suit a sprinter, so he showed his form," White said.

"He's had a rough last week. To do that today against those guys was a very impressive effort."

White also hailed the team's leadout for Ewan, calling it "standard" for them.

Sagan's Australian Bora-Hansgrohe teammate Jay McCarthy was third at Stirling and the Slovakian finished fourth.

Overall, Ewan leads Impey by 10 seconds and Sagan is third at 12 seconds.

Defending champion Richie Porte is equal sixth at 16 seconds, with the decisive stages to come.

The stage was raced in 30-degree-plus conditions and the temperature is forecast to soar into the 40s over the next two days.

Thursday's third stage from Glenelg to Victor Harbor has been shortened by 26km to 120.5km because of the extreme heat - a common feature through the Tour's 20-year history.

Norwegian Daniel Hoelgaard did not start stage two after crashing on Wednesday, while Swiss Steve Morabito managed to finish at Stirling despite hurting his shoulder in a mid-race crash.

This story Caleb Ewan wins hot second Tour Down Under stage in hills first appeared on The Sydney Morning Herald.