Jordan Spieth was lucky to still be just four shots behind.
"I felt like I was leaving a lot of shots out there".
Simpson, the 2012 U.S. Open champion, had to overcome a double-bogey at No. 13 after hooking his drive.
"I get done and look back and I'm surprised we're at 3-under", he said. Only the top five golfers in the FedEx Cup standings can automatically be the champion with a win.
The top five seeds only have to win the Tour Championship to claim the FedEx Cup, though there have been three occasions where someone else won at East Lake and walked away with golf's biggest payoff when the leading players faltered.
There's no guesswork involved with Spieth, the No 1 seed.
Anybody who thinks the money on offer here at the FedEx Cup is already absurd should stick around.
Of course, with two rounds to play there is far too many permetations and with Casey happy at least the likes of FedEx Cup front-runner, Jordan Spieth back at three-under with scores of 67 and 70.
There would be no better time to end the drought, but fellow Englishman Justin Rose also has the scent of victory, after a brilliant birdie at the final hole put him in a four-man pack just a shot off the lead.
"I'm just trying to play the best I can and win a tournament".
Thomas made birdies with a wedge in his hand, failed to save par from a bunker on the fifth hole, and then turned a mediocre round into a solid one with that 5-iron into the 18th for birdie. Both overcame a few stumbles on the back nine to share the lead.
"It would cap off a very good season, make a very good season a great season", he said. He ultimately finished his round with five consecutive pars.
Leishman ended his round on a high note with a long, curling putt for birdie at the par-5 18th but it was one of only two on the day against three dropped shots.
"These results over the years, you're up against 29 other guys so it's...but it's stills strong", he said.