Sitting in sixth spot entering the final round of the CJ Cup in Las Vegas, Day was finishing up his warm-up just moments before teeing off hoping to reel in American leader Russell Henley.

But after ripping his last few warm-ups, Australia's former world No.1 felt a slight twinge in his neck before his neck completely locked up on the short walk from the range to the practice putting green.

In pain but without time to get treatment, Day attempted to soldier on, only to hit his opening drive one-handed and into the rough en route to a triple-bogey seven on the first hole.

He eventually called it quits after hitting his second tee shot into the rough, then shanking his ball dead sideways.

Day's walk-off was the 11th time the 32-year-old has been forced to withdraw mid-tournament and comes less than a month before the Masters at Augusta National.

He went for treatment and remains cautiously hopeful of still taking his place at this week's ZOZO CHAMPIONSHIP at Sherwood Country Club in Los Angeles.

"My back had been a little jammed up and hurting most of the week so I think maybe I did something to my neck while trying to compensate for that," Day said.

"Hopefully it won't be too bad and I'll be back next week. I've been swinging it really well just needed a few more putts.

"It just sucks to have it happen any time but especially when you have a chance to win."

American Jason Kokrak eventually reigned at Shadow Creek, finishing at 20 under for the tournament and two shots clear of countryman Xander Shauffele following his sizzling 64 in the final round.

Day teed off in the third-last group with Kokrak, just two strokes behind the winner.

Australian Associated Press