Swede Henrik Stenson is threatening Jason Day's grip on the $US10 million Fedex Cup cash pool after the opening day of the Tour Championship in Atlanta.
A triple bogey seven at the par four fifth hole ruined Day's opening round.
The Australian had begun with three consecutive birdies to continue his dazzling form.
But his drive on the fifth sailed out of bounds and he gave back three shots to the field.
Day signed for a one-under par 69 and is tied-11th overall.
"I wish I could say that I was a machine and I can hit it straight down the gut every single time, 315 yards," Day said. "But unfortunately, as humans we're going to make errors. … To make triple was pretty disappointing, but it's early on in the tournament, so I can't dwell on that."
Stenson, who is ranked fourth on the Fedex Cup points list, carded a seven-under par 63 to launch himself into the top position on the projected standings, just ahead of Day.
The Swede is one of the five players who are guaranteed to win the Fedex Cup and its huge bonus pool if he wins the tournament this week.
One major positive for Australia on the day was the form of Queenslander Steven Bowditch, who is tied-fifth after the first round after an opening 68.
"It's tough out there. There's not much room for error. I guess that's what you get when you get to the last round of the playoffs," Bowditch said.
"Some of the guys out there are playing really well, but par's actually a pretty good score. Not being many par-fives there's not a lot of opportunities so when you get them you got to take them."
Leaderboard
1 Henrik Stenson -7
2 Paul Casey -5
T3 Rory McIlroy -4
T3 Zach Johnson -4
T5 Steven Bowditch -2
T11 Jason Day -1