Strip away the fame and fortune, and they are just regular buddies who love to play golf and play pranks on each other.
The pair, ranked two and four in the world respectively, have both had stellar seasons with a major title apiece.
But while there is little to separate them on the course, there is a clear winner when it comes to larking around, they told CNN Sport at the season-ending Tour Championship in Atlanta.
Spieth: "I would say me because I'll take things too far. You agree?"
Thomas: "Yeah, I kind of learned the hard way to stop doing anything to him because like he said he gets a little..."
Thomas: "Yeah, a little aggressive. He moved my car in a parking lot to a couple of lots over during a pro-am and I couldn't get hold of him and I couldn't get anywhere so that wasn't very fun."
'Act our age'
They have competed against each other since they were 13 years old, but Spieth made his major breakthrough first, winning both the Masters and the US Open in 2015 on the way to becoming world No.1.
This season he clinched a third major title with victory at the Open in July, but Thomas followed that a month later with his first major win at the US PGA Championship. He added a fifth PGA Tour victory of the season in early September.
But despite their success, both Spieth and Thomas are looking forward to some down time in the offseason.
"We're still 24. We need to act our age sometimes," says Thomas. "We have to let loose. We have to have fun. We're very lucky and very fortunate to get to do what we do and it's a very unbelievable, great life but it can get tiring and it's a grind.
"It's a lot of weeks on the road. A lot of weeks in new hotels, new beds, the media obligations, practising, making sure you have the right rest."
"The offseason is huge. To just hang those clubs up and do nothing and just to enjoy some time with our family and our friends -- just be us for a little bit."
The pair enjoyed some time off midway through the season when they went to the Bahamas for spring break with fellow pros Rickie Fowler and Smylie Kaufman.
Spieth has enjoyed developing friendships on tour and hopes the trips continue.
"It's an opportunity that we probably don't really get with our friends back home in jobs Monday to Friday and weekends off," Spieth says.
"Our buddies out here experience life kind of the same way we do. We have a lot more similarities within our job that we can share, talk about versus friends at home. You know it's kind of nice to have both -- that's what I'm getting at."
It's fair to say both men have earned a rest -- from golf, if not messing about like 24-year-olds.