WASHINGTON (AFP) - World number one Jason Day walked out of the USPGA Tour Championship in the second round on Friday with the back trouble that forced him out of the BMW Championship two weeks earlier.

Day came into the US$8.5 million (S$11.5 million) season-ending event, where a US$10 million bonus to the winner of the FedExCup playoff series is also up for grabs, cautiously optimistic that he could not only make it through but challenge for a win that would at least give him a chance of the bonus.

Despite "a bit of a sharp pain" on some drives he battled to a three-under 67 on Thursday that left him one shot off the lead.

He was even for the day through seven holes on Friday after a bogey at the fifth and birdie at the sixth.

At the eighth he found the water off the tee. He opted to use his driver out of the rough but was in the rough again when he called it a day, picking up his ball and shaking hands with playing partner Kim Si-Woo of South Korea before walking to the clubhouse.

Day, 28, finishes the season with three US PGA Tour victories, at the Arnold Palmer Invitational, the WGC Match Play and the Players Championship.