Adam Scott continued his FedEx Cup surge as defending champion Jason Day struggled badly in a rain-interrupted start to the US PGA Tour's penultimate playoffs event.
Fresh off fourth placings in the opening two playoff events, Australian Scott jumped to four under par through 10 holes in the BMW Championship before bad light ended played about 8pm at Crooked Stick in Carmel, Indiana.
The former world No.1 was three shots behind leader Roberto Castro, who shot a seven-under 65 to lead by one shot from fellow American Brian Harman.
Earlier, countryman and current world No.1 Day had six bogeys, including his first two holes, in a one-over 73 which left him tied for 51st in the 69-man field.
Day and Scott are both striving to become the first Australian to win the FedEx Cup and its $US10 million bonus, with Day currently second on the points standings and Scott fifth.
Both men are assured of places in the 30-man season finale, the Tour Championship, in two weeks but they need to remain in the top five to have a good chance of landing the $US10 million.
Scott benefited from not starting his round before a three-and-a-half-hour rain delay, coming out in soft conditions.
After three opening pars he rattled off three birdies and an eagle in his next six holes to make the turn five under.
An annoying three-putt bogey took a little gloss off but he was happy with his start.
"I played well hit some nice shots but I have to keep it going tomorrow when we return early," Scott said.
Aaron Baddeley is the next-best Australian at one under through eight holes while Marc Leishman sits even through 16.
Both Baddeley and Leishman need to finish in the top three to secure a place in the Tour Championship.
