Adelaide, Australia  - Australia's Allan Davis fancied his chances of keeping the race leader's ochre jersey in Friday's 143-kilometre fourth stage of the Tour Down Under.

"If the chance arrives, I'll definitely have a crack, but the plan is to keep it simple," the 26-year-old said on the starting line.

And the chance did come: The Quick Step rider won, flashing over the line ahead of Thursday's stage winner, Rabobank's Graeme Brown. Spain's Luis Leon Sanchez, from the Caisse d'Epargne team, took third place from local hero Stuart O'Grady, the Saxo Bank rider.

It was Davis' second stage win in the six-day Australian classic, which was split wide open by Thursday's carnage.

Germany's Andre Greipel was forced to retire after dislocating his shoulder when he crashed into a stationary police motorcycle at 55 kilometres per hour.

Greipel, 27, the defending champion and race leader going into Thursday's stage, is to fly to Hamburg for further treatment for injuries that were expected to keep the Columbia rider out of the sport for at least three months.

Seven-time Tour de France winner Lance Armstrong, in his comeback race after three years in retirement, finished in the bunch - as he has each stage.

Going into the 143-kilometre stage, the 37-year-old Texan was 29 seconds down on Davis and Brown in 39th place.

Armstrong said his real focus was on next month's Tour of California and eventual participation in July's Tour de France.

"Me being here is all about my own expectations and reminding people that it's a long season and that it's been three and a half years since my last race," he said. "That's a long time."

He tipped either Davis or O'Grady as the likely winner of the 11th edition of the Tour Down Under. (dpa)