Scotland's Oscar Onley mastered some of the WorldTour's premier climbers to win the Tour Down Under's gruelling fifth stage up Willunga Hill on Saturday, only to lose the overall lead on countback to Welshman Stephen Williams.

The 21-year-old DSM-firmenich PostNL rider outclimbed the leading race contenders for his first professional win ahead of Williams (Israel-Premier Tech) with Ecuador's Jhonatan Narvaez (Ineos Grenadiers) third.

Both Onley and Williams were credited with the same overall time of 16 hours, eight minutes and 18 seconds, but it was the Welshman who took over the leader's ochre jersey on countback heading into Sunday's sixth and final stage of the season-opening event.

"It's pretty incredible, I knew I was in good form coming here and I really wanted to do something on these final climbing stages, but against these guys, I wasn't really sure," Onley said.

"It's quite an iconic climb and to put my name on there is very special.

"I am going to have a lot of work to do tomorrow, but I have some really good teammates with me and I'm sure they will help me out."

Williams appeared annoyed as he just missed out on taking out the stage in a legs-pumping surge to the finish and later said he had misjudged the distance to the line from around the final bend.

The Welshman will have the prestige of taking the leader's jersey into the deciding stage of the Tour Down Under.

"It will be a tough stage tomorrow, it will be hard to control, everybody's still very close on GC (general classification), the top five or six guys all within a handful of seconds," Williams said.

"I expect a pretty stressful day, but the guys I have here are so experienced and class that I have no worries that we will do our best to fight for it."

It was a brutal day with two climbs up the famed three-kilometre (1.86-mile) Willunga Hill, with an average gradient of 7.4 percent.

Onley clocked in a cracking time for the 129.3-kilometre stage of two hours 52 minutes 23 seconds, representing an average speed of 45 km/h.

France's double world champion Julian Alaphilippe (Soudal Quick-Step) looked poised to take victory up the brutal Willunga Hill only to be overtaken by Onley, Williams and Narvaez inside the final 300 metres. He now lies fifth overall, 13 seconds behind the lead.

The overnight leader, Mexico's Isaac del Toro Romero (UAE Team Emirates), dropped back to fourth in the general classification, five seconds off the lead.

Onley showed incredible grit and stamina to take charge around the final bend and hold off the fast-finishing Williams to set up an enthralling final stage, which will play out up the 710-metre (2,330 ft) Mount Lofty, just outside of Adelaide.

"We had a look at the route last week. Tough circuit, demanding circuit. It looks like it's gonna be hot again as well," Williams said of the challenge confronting the peloton on Sunday.