Patrick Bevin rode well to retain the overall lead, but riders such as Porte, defending champion Daryl Impey and Michael Woods were prominent in a pulsating finish to the 146.2km stage in the Adelaide Hills.

Sagan, the three-time world champion, showed his class to overtake Spaniard Luis Leon Sanchez in the reduced bunch sprint and claim the stage.

It is the second-successive year that the Slovakian Bora-Hansgrohe ace has won in Uraidla.

While the weather at Uraidla was nowhere near as bad as last year's blast furnace, some riders tried to have the stage shortened.

But only moments before the start at Lobethal, a small minority of team directors voted against a proposed change.

After two days for the sprinters, Thursday was arguably the hardest stage in the Tour's 21-year history.

It featured seven laps of the tough 14km finishing loop and the peloton predictably detonated inside the last 20km.

The field was whittled down to a front group of 37 that featured all the overall contenders.

Woods (EF Education First) launched a blistering attack with 2km left and while it was nullified, Porte took careful note.

"That was a hard day. It was so nervous," the Australian Trek-Segafredo team leader said.

" If there's one thing to take out of it, it's that Woods is the strongest. He did a good attack there."

Porte now wants to see how Woods fares on Friday, with the steep Corkscrew climb only a few kilometres from the finish.

While Porte only finished 31st, he was in the front group and so lost no time.

He was also prominent near the finish before Woods attacked.

As Sanchez (Astana) narrowly missed a stage win for the second-successive day, Impey finished third.

That gave the South African a four-second time bonus, which is vital to his title defence.

His Australian Mitchelton-Scott team rode superbly, driving a fearsome pace in the last few kilometres after a surge from Team Sky had eliminated sprinters Caleb Ewan and Elia Viviani from contention.

Unlike pure climbers such as Porte, Impey is more of an allrounder and he needs the time bonuses.

He is fifth overall, 15 seconds behind Bevin, who was fifth in Thursday's stage.

"I'm a lot happier than if we finished fourth, that's for sure," Mitchelton-Scott team director Matt White said.

"We needed to take time today and it would have been a failure if we hadn't.

"As far as the team's performance, I was really happy."

Australian Associated Press