American Lee Williamson has upstaged the big-guns of Australian golf with a remarkable seven-under par 65 on day one of the MFS Australian Open in Sydney. Playing in the penultimate group of the day, Williamson rattled off six birdies in his opening eight holes to take the outright lead before a horror triple-bogey on the tough par-four ninth threatened to derail his round. But the man ranked 611th in the world would not be denied, carding four more birdies on the homeward nine to claim a two-shot lead. Robert Allenby, Andrew Bonhomme and Kim Felton lead the chasing pack after posting impressive rounds of 67. Former US Open champion Geoff Ogilvy, Scott Hend and Paul Marantz all sit a further shot back ahead of a cluster of players on three-under. In all seven players posted opening 69&aposs including American young-gun Brandt Snedeker, Korean Lee Sung along with Australians Stephen Leaney and Greg Chalmers. Pre-tournament favourite Aaron Baddeley mixed five birdies with three bogeys for an opening effort of 70. The ever-consistent Nick O&aposHern is also at two-under with Jarrod Lyle and Wade Ormsby who also made a solid start in tricky conditions. Last start winner Peter Lonard battled to a respectable one-under par round of 71, aided by three birdies in his final four holes. Defending champion John Senden is also among the cluster of players at minus-one along with former Open winner Stuart Appleby. Earlier, Bonhomme, who will ply his trade on the Nationwide Tour next season, posted eight birdies on the day to join the two-time champion atop the leaderboard. But it was Allenby who set the early pace with six birdies in seven holes midway through his round setting the tone, before his only bogey of the day brought him back to the chasing pack. Geoff Ogilvy, who partnered Allenby in the first group of the day, also took advantage of the benign early conditions and looks set to mount a serious challenge this week.