Dustin Johnson edged closer to the USPGA Tour's $10 million FedEx Cup playoff prize Friday, firing a three-under par 67 to seize a one-stroke lead in the season-ending Tour Championship in Atlanta, Georgia.

US Open champion Johnson, who arrived at East Lake Golf Club atop the playoff standings after his win at the BMW Championship two weeks ago, fired five birdies with two bogeys for a seven-under total of 133.

He was one shot in front of fellow American Kevin Chappell, who started the day in a three-way tie for the lead with Johnson and Hideki Matsuyama and signed for a second-round 68 for 134.

It was three shots back to Matsuyama, who shot a one-over par 71, and Kevin Kisner (70) on 137.

World number one Jason Day's season ended in disappointing fashion as the Australian walked off before completing the eighth hole with back pain.

It's the same injury that forced Day out of the final round of the BMW Championship, although he had voiced cautious optimism that his back would allow him to challenge for the win this week.

A statement from Day's management team described the injury as a "strained ligament in his lower right back with a muscle spasm" and his withdrawal as a "precautionary measure".

The top five players in the playoff standings coming into the $8.5 million season finale can each claim the bonus with a victory.

Anyone lower in the standings must win and count on those top players finishing down the leaderboard -- something that was looking less and less likely in Johnson's case.

- 'Really solid' -

Johnson, already the front-runner for player of the year honors, is seeking his fourth title of 2016.

"I felt like I played really, really solid today and just didnt' really get a lot out of it," Johnson said. "I had some close looks that I didn't make.

"Still, obviously three-under is a great score around here. I'm never going to be upset with that."

Johnson had back-to-back birdies at the fifth and sixth and again at 10 and 11 before is first bogey of the day at the 12th. He nabbed the first birdie of the day at the par-four 16th, hitting his approach from 147 yards to 12 feet and draining the putt.After giving a shot back at 17, Johnson saved par from six feet at the last, where Chappell missed a chance to pull level.

Chappell, seeking his first PGA Tour title, had three birdies in a row from the sixth through eighth. He bogeyed the 11th, but stayed steady coming in to keep himself in contention.

England's Paul Casey, Northern Ireland's Rory McIlroy and American Ryan Moore were tied for fifth on 138.

Scotland's Russell Knox carded the low round of the day of 66 to join South Korea's Kim Si-Woo and American Justin Thomas on 139, while defending champion Jordan Spieth was in a group on 140 that also included Australian Adam Scott.

World number seven Bubba Watson, angling for the final captain's pick spot on the US Ryder Cup team, posted an unimpressive 73 that left him tied for 25th.

Source: AFP