A five-time victor this season, he hit his approach shot on the 590-yard No. 18 to within six feet of the cup and made the putt for eagle.
Spain's Jon Rahm, who is fifth in the points list, is one of four on six under after his 67, along with England's Justin Rose (66) and American duo Patrick Reed (65) and Gary Woodland (67).
For Day the round got off to a great start and at 4-under through 10 holes and just one off the lead he looked to be stamping his authority on the season ending tournament.
It is Thomas' second appearance in the Tour Championship, his only other coming last season, when he finished in a tie for sixth. Holing that putt made this feel like a pretty good day after all.
In addition to who might win the FedEx Cup, one of the more intriguing storylines entering the Tour Championship was the player of the year race. "It was about as much as I can get out of a five iron", he said, before adding that he was trying to keep thoughts of the FedExCup out of his mind.
Madrid, Barcelona clash over control of Catalan police
They also arrested 14 organizers of the vote, including several top Catalan government officials. Visitors to Barcelona are being warned of demonstrations in the city that could disrupt travel.
Dustin Johnson and Jordan Spieth are four shots back.
Unable to make many putts, the reigning PGA Championship victor played his next 11 holes in even par and reached the 18th tee two shots off the lead.
Dufner was the last player to make it into the Tour Championship field, but he is just two shots back of the three co-leaders heading into the weekend. Both overcame a few stumbles on the back nine to share the lead. Since then he has claimed 15 top five finishes - three more than any other "win-less" player has managed in the same period.
"Today was very pleasing, I probably played better than I did yesterday though it was all going to plan playing tidy golf except for those blemishes on the card were from some sloppy stuff", said Casey.
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.