MELBOURNE (REUTERS) - Unheralded local player Nick Cullen upstaged defending champion Adam Scott, by shooting a sparkling six-under 65 to take a two-shot lead after the opening round of the Australian Masters on Thursday.

The 29-year-old Cullen, ranked well outside the world's top 300, grabbed a share of the lead with compatriot Scott after a sublime chip-in for eagle on the par-four 11th, then left the US Masters champion in his wake by rolling in three more birdies on a chilly, blustery day at Royal Melbourne.

Having long lived in the shadow of twin brother Dan, an elite cricketer who played a Test match for Australia, Cullen was tickled to see his name at the top of the leader board above the headline act.

"I think it's an amazing golf course and I'm really loving it here this week," the Adelaide professional told reporters after offsetting two bogeys with six birdies at the famed sandbelt course.

Enjoying a rich vein of form, Scott teed off early and carded a solid four-under 67 to sit tied for second with German Maximilian Kieffer.

The world No.2 caught fire with four straight birdies in the middle of his round, thrilling hundreds of fans who stuck by him through a drizzly morning, but he marched straight to the practice range to bash a few dozen balls in frustration after his ball-striking wavered in the closing holes.

"Sixty-seven around here is a good score," Scott said after a five-birdie round marred only by a solitary bogey on his second-last hole.

"I felt like I left a couple out there but my game is in pretty good shape and four rounds like that might go a long way this week."

Having won the Australian PGA Championship at a canter on Sunday, the 33-year-old remains well-placed to sweep the country's three marquee tournaments, with the national Open title in Sydney to come.

Little-known Kieffer made a mockery of the adage that practice makes perfect as he joined Scott on four-under despite having never played the course's back nine.

On his first trip Down Under to play the World Cup of Golf for Germany, the 23-year-old missed out on a hit on the back nine in the lead-up owing to jet lag and poor weather, but relied on caddy Graeme Heinrich, a Melbourne man and publisher of golf course yardage books, to talk him through the layout.

"My expectations were very low because my preparation was not as it should be but it was good fun and a good learning experience," Kieffer said.

World No.8 Matt Kuchar, who will defend his World Cup of Golf title for the United States, this time with Kevin Streelman, at the same course next week, is six behind Cullen after battling to an even-par 71.

Former world No.1 Vijay Singh cancelled out four birdies with five bogeys on the way to a scratchy 72 to be a further stroke adrift.