The best friends head into the Tour Championship in Atlanta ranked first and second in the race for the PGA Tour's season-long FedEx Cup.
But despite the bumper jackpot on offer for the winner of the overall FedEx standings, they say it is not the big bucks that motivate them.
"We don't do this for the money," Thomas, 24, told CNN World Sport. "It's just an added bonus. This is what we love -- we turned pro and left college early to try to win tournaments, win majors, win FedEx Cups."
The pair have certainly enjoyed incredible success this season. Thomas has five victories to his name, including his first major title at the US PGA Championship, while Spieth clinched the third major of his career at the Open at Royal Birkdale in July.
"The bonus that comes with the FedEx is nice and all, but a major championship cements you in the history of the game," said the 24-year-old Spieth, who has amassed nearly $35 million in his short career.
"The 18th greens of our majors. Seeing the putt actually go in -- the exhale that comes out from the highest pressure in the game. That's why we play. That's what it's about."
World No. 2 Spieth and fourth-ranked Thomas have known each other since they were 13 years old, and the now-famous picture of them sat together as juniors is one of the most circulated sporting images of 2017.
Their friendship remains as strong as ever -- for the last two years they have enjoyed spring break together in the Bahamas, and they shared a rented a house for the Open at Royal Birkdale with fellow pros Rickie Fowler, Jimmy Walker, Zach Johnson and Jason Dufner.
It is clear they enjoy sharing in each other's success. Thomas missed the cut at the Open, but stayed on and was greenside to congratulate Spieth on winning the Claret Jug after a thrilling final-round battle with Matt Kuchar. Spieth returned the favor three weeks later at the US PGA at Quail Hollow.
"You don't realize how much it means to you until you have one of your friends stay around and watch," said Thomas, who in January became the seventh player in PGA Tour history to shoot a 59.
"When I finished and had him, Rickie [Fowler] and some other friends and their girlfriends sticking around, that stuff's pretty special and it's pretty cool."
For the Tour Championship, FedEx Cup points accrued over the season are reset, meaning mathematically any of the 30 competitors in the field can win the title and the $10 million bonus.
Behind Spieth and Thomas lurks world No. 1 Dustin Johnson, and the pair know it is likely to be a winner-takes-all shootout at East Lake Golf Club.
"We both have a great opportunity to do something special this week," said Thomas. "We're both treating it as any other major event. We both had unbelievable years. I feel like it would kind of turn a good year into a great year and we both have the opportunity to kind of put the cap on it."
Spieth previously claimed the FedEx title back in 2015 -- the season which saw him surge to top of the world rankings with back-to-back major victories at the Masters and US Open. Thomas could only watch and admire back then, but things have changed and he is happy finally to compete with his buddy on the biggest stage.
"His accomplishment and everything he's achieved is a lot more and higher than anything I've done," Thomas says. "But it's fun to have that, you know?
"It keeps me working, it keeps me pushing to try beat him because I feel like if I'm beating him in a tournament, I will then have a good chance to win."
Regardless of who is celebrating come Sunday evening, both Spieth and Thomas agree that it will not define their seasons and that their respective major triumphs will live longest in the memory.
But $10 million -- plus a Tour Championship winner's cheque for $1.5 million -- buys an awful lot of bragging rights.