A back injury prompted world number one Jason Day to pull out of the Tour Championship during the second round in Atlanta on Friday.

Day withdrew while playing the eighth hole at East Lake.

The Australian picked up his ball in the rough, shook hands with fellow competitor Kim Si-woo and walked back to the clubhouse.

Day, 28, also withdrew during the final round of his previous tournament, the BMW Championship two weeks ago.

Before the start of the Tour Championship, Day said his back was an on-going problem due to a bulging disc that caused occasional flare-ups.

He also said after Thursday's opening round that he had felt pain on a few shots, but not constantly.

"There was a couple of drives out there where ... felt a bit of a sharp pain in my back," he said after the round.

Day's withdrawal eliminates him from contention to claim the $10 million bonus awarded this week to the winner of the PGA Tour's season-long FedExCup points race.