(Reuters) – Jason Day had a new caddie to carry his clubs but was stuck with the same old troublesome back in the opening round of the BMW Championship on Thursday.

Walking gingerly between shots at Conway Farms outside Chicago, the former world number one nevertheless swung with apparent freedom to card a seven-under-par 64 and end the day two strokes behind compatriot Marc Leishman in share of second.

“It’s just a little stiff in the back,” said the long-suffering Australian, who has been plagued by recurrent pain throughout his career. “I feel I have a stiff back every week.”

After dumping Col Swatton in the run-up to the tournament, Day strode the fairways in the company of childhood friend Luke Reardon, who was working as a fill-in caddie this week.

Day says Swatton, his mentor, coach and caddie since the age of 12, will stay on as instructor.

”Luke did a fantastic job out there today,“ Day said. “You never know whether you’re going to gel. He’s a good golfer, has got a keen golf I.Q.”

Day has happy memories of Conway Farms, where he gained the world number one ranking by winning the BMW Championship two years ago.

He is 40-under par in his career at Conway Farms, nine shots better than any other player, but he has not been his usual self for much of this year.

Something had to give and, as with Rory McIlroy and Phil Mickelson before him this northern summer, it was the caddie.