Kevin Streelman and Thomas Bjorn shot 5-under 66s Thursday to share the lead after the first day of the World Cup.
Streelman birdied five of six holes on the back nine at Royal Melbourne, but the American then finished with two late bogeys.
K.J. Choi, Martin Laird and Stuart Manley were tied for third, one stroke behind Streelman and Bjorn.
Another American, Matt Kuchar, had four bogeys on the back nine for 71. The Americans and Denmark were tied for the team lead, three strokes ahead of Portugal.
Adam Scott, who won the Australian PGA and Australian Masters over the past two weeks, had a quintuple-bogey 9 on the 12th hole and shot 75.