Jason Day has pulled the plug and withdrawn midway through the second round of the US PGA BMW Championship.

World No.6 Jason Day has withdrawn from the BMW Championship with a back injury after battling through nine holes of the second round.

Day, who first injured his back in the Wednesday pro-am and carried the problem into the opening round, decided to play on Friday after an MRI cleared him of serious damage.

But the lower back strain continued to bother him and, after hitting his approach on the ninth hole, the Queenslander pulled the pin.

He had been struggling at three-over on his round and decided to try to protect himself for next week’s season-ending tour championship, although his participation at East Lake is still uncertain.

“I am extremely disappointed to have to pull out of this event,” Day said.

“I will be getting treatment over the next few days and hopefully be ready for the Tour Championship.”

Day, who entered the penultimate playoff event at seventh on the points list, is currently projected to drop to 11th.

He could end up anywhere from seventh to 15th heading to Atlanta should he be able to take his place, where the overall winner claims the $US10 million ($A10.82 million) bonus.

Before he started his Friday round, Day described his injury as just normal wear and tear.

“The MRI came back clear. I just have some stiffness and soreness. I guess you could just say it is golfer’s back,” Day told AAP.

“It turns out my legs are too strong for my core and it is adding stress to my back, so I really have to strengthen my core over the off season to try and lessen the impact.”

This is just the latest in a series of injury and health problems which have blighted Day’s season.

He’s had a horror run since incurring thumb injury while winning the WGC-Match Play Championship in February.

He was able to play just four tournaments over the next five months, with several throat infections ruling out earlier comebacks.

When at last able to return to full practice at the WGC-Bridgestone Invitational in early August, Day was floored by a bout of vertigo, a side effect to his medication, forcing another withdrawal.

Finally hitting his straps in the last month with a tie for 15th at the US PGA Championship, a second place at the opening playoffs event and a seventh at the second one last week, Day was understandably frustrated with this latest complaint.