Jason Day has been rocked by injury during the opening round of the lucrative US PGA Tour Championship but this time it is caddy and coach Col Swatton who has been forced to withdraw.

In an ironic twist Day was under a back injury cloud himself entering the event but, after seven holes in the chase for the FedEx Cup, he was playing without the only caddy he has ever known after Swatton was forced off the course.

Swatton had woken with a locked back but, after being treated by a trainer who diagnosed a problem with his sacroiliac joint, he took his place on the first tee.

After seven holes at East Lake Golf Club the pain was too much and mental coach Jason Goldsmith took charge of Day’s bag.

Swatton, who took Day under his wing at high school in Queensland and nurtured him all the way to the PGA Tour, is hopeful of returning for the second round once the swelling reduces.

Through nine holes of the opening round Day was one-under par and tied fourth just one shot off the lead.

Day started the tournament in 10th position on the season long points list, needing to win the event to have any chance of winning the FedEx Cup and the $US10 million ($A10.82 million) bonus.

To take out the obscene cash prize Day also needs Chris Kirk (No.1) to finish fifth or worse, Billy Horschel (No.2) to finish in a three-way tie for third or worse, Bubba Watson (No.3) finish in a three-way tie for second or worse and world No.1 Rory McIlroy (No.4) must finish T2 or worse.

Americans Brendon Todd (15 holes), Ryan Palmer (14 holes) and Patrick Reed (12 holes) led the way at two-under par.

World No.2 Adam Scott and John Senden were the next best Australians at one-over in a tie for 17th while Geoff Ogilvy was struggling in 29th and last place at six over.