CHICAGO (AFP) - World number one Jason Day of Australia withdrew Sunday from the BMW Championship due to left back pain after hitting his ninth-hole tee shot at the penultimate US PGA playoff event.
The defending champion was three-under for the tournament and two-over for the day at the 7,516-yard Crooked Stick layout in suburban Indianapolis.
Day said he felt the pain after hitting his tee shot on the last hole of the front nine.
He decided to withdraw rather than risk greater injury, but the 28-year-old Australian is expected to play in two weeks in the Tour Championship, having safely qualified to be among the 30 who advance to Atlanta.
Day's victory at last year's BMW boosted him atop the rankings for the first time only weeks after his first major triumph at the 2015 PGA Championship.
This season, Day won the Arnold Palmer Invitational and a week later captured the World Golf Championship Match Play crown.
Day followed in May with a victory at The Players Championship and last month was runner-up to American Jimmy Walker in defending the PGA Championship title - Day's seventh top-10 finish in his past eight major starts and 13th major top 10 in 25 major starts.
On Sunday, Day made a bogey at the second hole and double bogey at the par-3 third before a birdie at the par-5 fifth helped lift his spirits.
The top 30 players in season points after this week's event advance to the Tour Championship, with the five points leaders able to capture the $10 million playoff bonus by winning the tournament.
Day stood second in points but rivals had the chance to bump him down the list and out of the all-important top five.