It was South African rival Daryl Impey, though, who assumed the overall lead with just one stage remaining of this opening International Cycling Union World Tour event of the season. 

Porte, the Australian rider representing BMC Racing, attacked with 1,500 metres remaining at the end of the 151.5 kilometres stage from McLaren Vale. 

He held on to finish eight seconds ahead of Mitchelton-Scott's Impey in 3 hours 42min 22sec and collected a four-second time bonus for winning on a day dubbed as the "queen" stage as it was the toughest test of the race.

Remarkably, this meant the two leaders were tied absolutely level in the overall standings as Impey had led by 12 seconds at the beginning of the day.

Impey assumed the lead on count-back because of second-placed finishes on stage two and four.

He is now poised to be crowned the winner with just the final flat stage to come.

Tom Jelte Slagter of The Netherlands and Dimension Data finished third a further two seconds behind.

He also moved into third place overall, 16 seconds behind the leading duo.

Slovakia's world champion Peter Sagan, yesterday's stage winner and the overall leader at the beginning of the day, slipped out of contention today before the final climb.