ATLANTA, GA - SEPTEMBER 23: Adam Scott of Australia hits his tee shot on the second hole during the second round of the TOUR Championship at East Lake Golf Club on September 23, 2016 in Atlanta, Georgia. (Photo by Kevin Cox/Getty Images)

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.