Williamson, a 28-year-old former collegiate player at Purdue, took over the lead late in the day, nearly six hours after Robert Allenby's 67 had put the Australian into a share of the lead.
Williamson birdied the 16th hole to move in front, then birdied the par-5 18th to give him a two-stroke cushion going into Friday's second round at The Australian course.
Williamson, a regular on the Canadian tour between 2003 and 2006, birdied his first three holes and three of the next five before stumbling with a triple-bogey 7 on the ninth hole, still making the turn in 33.
'I scored better than I played,' said Williamson, who finished 84th on the Nationwide Tour money list this year.
'I putted well, I pitched in twice and feel like I hadn't pitched in for years. I'm glad I'm going back out early tomorrow and I'll just try to continue what I've got going.'
Allenby and two other Australians -- Andrew Bonhomme and Kim Felton -- were tied for second.
'I'll take 5 under around here any day,' Allenby said. 'It's a tough golf course with the wind blowing. I made a few good putts early, and a few good saves.'
Bonhomme, a 35-year-old journeyman who qualified for the Nationwide Tour next year, had a morning start along with Allenby, and Felton played in the afternoon.
Australians Geoff Ogilvy, Scott Hend and Paul Marantz were tied for third with 68s, and American Brandt Snedeker and South Korea's Lee Sung were in a group of seven with 69s, four strokes back. Aaron Baddeley and Nick O'Hern were in a large group at 70.
Allenby, who won the tournament in 1994 and 2005, had six birdies with a bogey at the par-4 7th.
Former U.S. Open champion Ogilvy had five birdies, including a 20-foot putt on the par-3 15th hole.
'All in all it was pretty good really,' Ogilvy said. 'I would've thought under par is a good score anytime you play here.'
Bonhomme, whose name was incorrectly spelled 'Bonholme' on his caddie's bib and on the leaderboard Thursday, has earned an average of $24,250 a year in 14 years as a pro -- many of them on minor Australian pro-am circuits.
He credits his recent stronger play with a switch to longer putter -- as suggested by his wife, Simone.
'My wife finally convinced me to go to the long putter just over a year ago,' he said. 'She probably deserves a few presents for it I guess.'
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