Both Australians fell victim to the par-4 13th, Day with a bogey while Leishman wracked up a disastrous double, Day also going on to bogey the par-5 18th.
Day fell from 3-under for his round to 1-under with the late mistakes and dropped from inside the top five to a share of 17th. Leishman ended the day T21 in the 30 man field.
On a day when many of the game’s biggest stars struggled, Stanley was unflappable with four consecutive birdies from the second hole seeing him to a healthy lead.
He faltered with a bogey at the eighth but three back nine birdies saw him sign for 6-under 64, enough to grab a two-stroke lead over Webb Simpson, Brooks Koepka, Paul Casey and Daniel Berger.
A group of four players sit a stroke further back sharing sixth, Jon Rahm and Jordan Spieth among them after 3-under scores of 67.
Day endured a roller coaster round on day one dropping two shots in his first five holes to be well off the pace early.
As has become his custom, though, he hit back with five birdies in his next six holes to claw his way back into the mix before the clumsy finish dropped him down the standings again.
Leishman’s day was less adventurous with one birdie and one bogey on the outward nine before the adventures of the back half where a birdie at 15 got back one of the two he gave away at 13.
At 1-over he is seven back and near the tail of field but with such a small field both he and Day will know they are far from out of the running.
Rahm and Spieth are the best of those in the top five on the FedEx Cup heading to the final event of the year, a win for either guaranteeing a win in the FedEx Cup.
Spieth started slowly with a bogey at the second but got back on track with three straight birdies from the sixth.
He told Golf Channel’s Steve Sands after the round it wasn’t the start he wanted but he was pleased with his fightback.
“I just wanted to have a few good swings in a row,” he said of his struggles over the opening five holes.
“I was kind of all over the place, I missed fairways left and missed them right early in the round and was just searching for something to go off of.
“Then hit a great shot on the par-5 into the green from the rough and made a birdie there and hit two great shots at the next and really struck the ball beautifully from there.”
He said despite being three off the pace he was pleased with his opening day’s work.
“3-under is a good solid round on this golf course,” he said.
“I mean, it really sneaks up on you. It can be difficult.”
Dustin Johsnon, the World Number One and third on the FedEx Cup to start the week, battled his game all day but signed for a credible 2-under 68.
He sits T11 but boasts a formidable record at East Lake Country Club in recent years with finishes of 5th, T5 and T6 in his last three appearances.
ALISON WHITAKER ON ESCAPING BAD LIES:
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