Casey's performance this week at East Lake shouldn't be a surprise to anybody.
"I'm bummed to not be in a play-off and have a chance to win, but that was a gutsy birdie Xander made on 18", Thomas - who won five tournaments in a breakout season - told Sky Sports.
The eagle came at the par-5 sixth hole where Casey sank a 11-foot putt.
There it remained until the 17th, where Casey superbly holed out from 40ft for a birdie.
The Australian is tied seventh and six shots behind leader Paul Casey, whose 65 rocketed the Englishman to 12 under and two clear of Kevin Kisner (63) and fellow American Xander Schauffele (65). "So it's going to be a tough one". The consolation prize for Casey is he gets to go home and see his newborn daughter for the first time. Rory McIlroy won previous year as the No. 6 seed. "I just have to keep up what I have been doing all week". "I was consistent. A lot of weeks and great memories".
But Casey failed to get up and down from greenside bunkers on the next two holes, while Kisner birdied the 13th and chipped in for another on the next.
Day, Simpson, Thomas and Casey are all 7-under.
It was the first time in eight years that two players celebrated at East Lake.
He subsequently moved to the United States and married a woman from Taiwan, the couple settling in southern California. I'm a little speechless. "That's quite a long stint without him winning because he is such a good calibre player".
"It's just huge and it's awesome just because I feel like I played so well this year", said Thomas.
FedEx Cup champion Justin Thomas says he'll lean on Tiger Woods and Jordan Spieth for advice on how to sustain his world-beating form after becoming golf's $25 million dollar man. This was one stroke better than the best single round score that Spieth had during the tournament.
Spieth used his 2014 and 2016 Australian Open victories as a springboard to claiming the 2015 Masters and US Open, as well as the British Open this year. Next week: Presidents Cup.
"I'm looking forward to a day without golf", Spieth admitted.
"I saw Brooks (Koepka) make the putt right before me and I thought "Yeah I'll just brush it in" and then I thought I'd missed it". I'm probably one of the most excited when it comes to enjoying what I do'.