Jason Day is battling a back complaint midway through the opening round of the US PGA tour’s BMW Championship, the penultimate playoff event in the battle for the season-long FedEx Cup.

Day is even par through 11 holes and tied 21st, but is in obvious discomfort holding his back after shots and even receiving some treatment from his trainer at the turn.

At seventh on the season long points list, Day is the highest ranked Australian, currently giving him the best chance of winning the FedEx Cup and the $US10 million ($A11 million) bonus.

The world No.6 sits four shots off the lead held by American Gary Woodland, who is four under through 12 holes.

Adam Scott, Stuart Appleby and John Senden are all tied with Day at even par as the leading Australians.

Geoff Ogilvy, needing a reasonable finish to solidify a spot to next week is one over through seven holes, while Matt Jones, who needs to be in the top three to advance is also one over and tied 44th.

Steven Bowditch and Marc Leishman, who need to finish in the top four to advance are already long odds, sitting in a tie for 58th at two over.