Despite finishing just two shots off the lead following the opening round at the BMW Championship in Illinois, Padraig Harrington feels he will need to improve over the weekend if he is to post a first win of the year.

Harrington, who failed to close out winning positions at the first two play-off tournaments, finished alongside Tiger Woods on three-under.

That was two behind Rory Sabbatini, who shot a five-under 66 to snap out of a form slump and earn a share of the first round lead with Steve Marino.

But Harrington said he was lucky to escape with a bogey-free round.

"In terms of scoring it was steady but it was far from steady on the course," he said after his three-birdie 68. "It was a bit erratic at times out there.

"I struggled to focus, so it needs to be better over the next three days. I won't get away with what I got away with today."

Sabbatini stumbled with a bogey at the first but notched five birdies in his last 12 holes to put two missed cuts at the first two PGA play-off events behind him.

In-form Australian Marc Leishman and American Bo Van Pelt trail the leaders by a stroke.

Sabbatini, currently 35th in the play-off standings, needs a solid finish here to be among the 30 players to advance to the Tour Championship finale, where a $10 million bonus awaits the play-off points champion.

At 26th in the standings, co-leader Marino is also not assured of qualifying, but after a slow start gave himself every chance by compiling five birdies in the final 11 holes.

Woods, second in the points standings, is guaranteed a place in the finale, and was pleased with his four-birdie one-bogey round.

"I hit the ball well," the world number one said. "I think anything under par is a really good score."

Collated first-round scores(USA unless stated, par 71)

66Steve Marino, Rory Sabbatini (Rsa)

67Marc Leishman (Aus), Bo Van Pelt

68Tiger Woods, David Toms, Geoff Ogilvy (Aus), Camilo Villegas (Col), John Mallinger, Padraig Harrington(Irl)

69Bubba Watson, Charles Howell III, Mark Wilson, Dustin Johnson, Anthony Kim, Ian Poulter (Eng), Brandt Snedeker, Jonathan Byrd

70Nick Watney, John Senden (Aus), Heath Slocum, Webb Simpson, Chad Campbell, Sean O'Hair, Jim Furyk, Bryce Molder, Luke Donald (Eng), Scott Verplank

71Brian Davis (Eng), Sergio Garcia (Spa), Matt Kuchar, Jason Day (Aus), Phil Mickelson, Charlie Wi (Kor), Bill Haas, Jason Bohn, Y.E. Yang (Kor), Ryan Moore

72Steve Stricker, Pat Perez, Retief Goosen (Rsa), Jeff Overton, Stewart Cink, Kevin Na, Fredrik Jacobson (Swe), Mike Weir (Can)

73Charley Hoffman, Zach Johnson, Brian Gay, Angel Cabrera (Arg), Hunter Mahan, Nathan Green (Aus), John Rollins, Justin Leonard

74Jason Dufner, Paul Goydos, Lucas Glover, Davis Love III

75Ben Crane, Robert Allenby (Aus), Ernie Els (Rsa), Kevin Sutherland

76Jerry Kelly, Woody Austin, Stephen Ames (Can)

77Kenny Perry, Bob Estes

78J.B. Holmes, Tim Clark (Rsa)