Jason Day's US PGA Tour season has come to a sad and abrupt halt, with the world No.1 forced to give up the fight for the championship and lucrative FedEx Cup with a recurring back injury.

Day withdrew through seven and a half holes of round two at Atlanta's East Lake Golf Club, giving up a chance to claim $US11.53 million and likely his shot at Player of the Year.

He was just three shots behind the lead but in the midst of making a mess of the eighth hole when he decided to pull the pin as back spasms continued to plague him.

The Queenslander suffered an annulus ligament tear in a disc in his back at the BMW Championship less than two weeks ago but attempted to chase the season-long title this week.

SOURCESydney Morning Herald