World No.1 Jason Day has sensationally withdrawn after completing just seven holes of his second round of the Tour Championship due to a back injury.

Day was just three shots off the lead but in the midst of making a mess of the eighth hole he decided to pull the pin, ending his chances of winning the season-long FedEx Cup and US$10million bonus.

The Queenslander withdrew with a ligament tear in his back at the BMW Championship less than two weeks ago and sat just one shot off the lead after his first round.

He carded a birdie and a bogey on Friday before things unravelled at the eighth when his tee shot found the water.


He then attempted to hit driver off the deck from the rough and found tree trouble.

After another shot left him off the green he picked up the ball and started walking into the clubhouse.

Meanwhile, Dustin Johnson continues his impressive form and remains in a tie for the lead.

Johnson is two under on his second round after eight holes to push to six under for the tournament, and is tied at the top with fellow American Kevin Chappell (seven holes).

Adam Scott is also struggling, three over after 11 holes to drop to two-over and into a tie for 17th in the 30-man event.