World number one Jason Day is in good shape after the opening round of his title defence at the first of four US PGA Tour Playoff events.

Despite battling a wayward driver, Day shot a 3-under-68 at Bethpage Black to sit just two shots off the lead midway through the opening round.

Already leading the season-long FedExCup race, Day is looking to become the first Australian in the 10-year history of the Playoffs to win it all at next month's Tour Championship, including the $US10 million ($A13.1m) bonus.

Adam Scott, who sits third in the points race, also started strongly with a solid 69 to be just three off the lead.

American Patrick Reed and Scotland's Martin Laird lead the way from the morning wave after posting in at 5-under 66.

Argentina's Emiliano Grillo and Americans Rickie Fowler and Kevin Chappell and JB Holmes share third at 4-under.

Day hit just five of 14 fairways in his round but managed four birdies and just a lone bogey.

"It was just a little bit I think more so rust, a bit of mental rust than physical rust," he explained after returning from a three-week break.

"I shouldn't say rust. It's more the sharpness wasn't quite there compared to when you're about two or three tournaments in.

"Overall, I felt great about how I played. Did a lot of good quality things out there, and hopefully I can just keep pushing that from the second to fourth round now."

Baddeley fades after strong start

Aaron Baddeley was an early leader with two birdies in his opening four holes before fading late.

Three bogeys in his next seven holes pushed him over par but the wheels came off when he dropped four more shots in his final four holes for a 5-over-76.

John Senden finished 1-under and Marc Leishman was even par to sit in a tie for 34th.

Senden is facing elimination this week if he cannot perform.

Meanwhile British Open champion Henrik Stenson has withdrawn with pain his right knee.

Stenson, who won the silver medal at the Olympics two weeks ago, opened with a 74.

He had surgery on the same knee during the off-season.

Stenson started the play-offs at number 14 and likely will fall out of the top 20. The top 30 advance to the finale at the Tour Championship.

AAP