A statement from Day's management company read: "Jason has a strained ligament in his lower right back with muscle spasm. He withdrew as a precautionary measure. Jason should be fine with some rest after a long break in the off-season."

The Australian was one of five players who began the week knowing victory at East Lake would also secure the overall FedEx Cup title, with Johnson, Patrick Reed, Adam Scott and Paul Casey the others in control of their own destiny.

Johnson held a share of the lead after the first round and edged a shot clear of the field at halfway after adding a 67 to his opening 66, thanks to five birdies and two bogeys.

Fellow American Kevin Chappell had carded a hat-trick of birdies from the sixth to briefly take the lead, but bogeyed the 11th - after receiving treatment for a bee sting - and had to settle for a 68 to finish a shot behind the US Open champion.

Even if Chappell wins his first PGA Tour title on Sunday, he needs Johnson to finish outside the top 10 to be able to win the 10million US dollar bonus as FedEx Cup champion.

American Kevin Kisner and Hideki Matsuyama were four shots off the lead on three under par, with Rory McIlroy, Paul Casey and Ryan Moore another stroke back.

McIlroy made a poor start with four bogeys and one birdie in the first eight holes, but the four-time major winner responded with birdies on the 10th, 12th and 17th to remain in contention for a third win of the season.

"I struggled with my swing all day, even from the warm up on the range," McIlroy told Sky Sports. "I just didn't feel comfortable with my swing and sort of let that feed in to how I played, especially the first few holes. I made some really sloppy swings and poor shots.

"I got it together a little bit on the back nine. It was disappointing not to make a birdie on the last [after a drive of 359 yards] but it seems like everyone is finding it quite difficult out there so I'm not too far away and if I can straighten it out over the weekend, hopefully I still have a chance.

"If I can shoot a couple of scores in the mid 60s at the weekend, you never know."

Scotland's Russell Knox, who had expressed his determination to win the FedEx Cup title after controversially missing out on a Ryder Cup wild card, was a shot behind McIlroy after adding a 66 to his opening 73.

"Yesterday I was atrocious," Knox said after four birdies in his last eight holes. "I didn't drive it well, I got unlucky a few times just missing the fairways in some terrible lies and hit some sloppy iron shots. Today I drove it a little better and my iron game was back to normal and I've been putting very well.

"It's a huge honour to be here. It shows how well you've played all year. I'm very proud of myself and hopefully this round has given me an interesting weekend. This round has obviously put me back in there. This course is very difficult and a couple more of those and you just never know."