Jason Day surprisingly stumbled after beginning his reign as world number one in brilliant fashion in pursuit of a fifth win in his last seven events in the Tour Championship in Atlanta.
Day overtook Rory McIlroy at the top of the rankings after winning the BMW Championship on Sunday, his second victory in three FedEx Cup play-off tournaments and fifth of the year.
The 27-year-old was 101 under par for his last seven events, including a major championship record of 20 under in winning the US PGA Championship at Whistling Straits.
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And the Australian looked to have carried on exactly where he left off on Thursday, carding birdies on the first three holes at East Lake before slicing his drive on the fifth out of bounds to run up a triple-bogey seven.
Top of the world
Day is top of the FedEx Cup standings and, along with Jordan Spieth, Rickie Fowler, Henrik Stenson and Bubba Watson, would secure the overall title and USD10million bonus with victory on Sunday.
Stenson could even claim the eight-figure payday for the second time in three years without winning all season, but the Ryder Cup star had also birdied the first three holes before picking up another shot on the sixth.
That left the 39-year-old Swede a shot behind early pacesetter JB Holmes, who had gone to the turn in 31 and picked up another shot on the 10th.
Playing alongside Day, Masters and US Open champion Spieth was one under par after a birdie on the second, with McIlroy one stroke better off after recovering from an early bogey with a hat-trick of birdies from the seventh.
McIlroy admitted on Wednesday that the bonus money "luckily, doesn't sort of mean much to me anymore" but insisted he was determined to add the FedEx Cup title to his CV after finishing second to Billy Horschel 12 months ago.
Is Day about to enter an era of dominance?
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