Johnson finished three shots ahead of 2017 FedExCup champion Thomas and Xander Schauffele at East Lake in Atlanta to win the Tour Championship and a AU$21-million payday.

But such success seemed unlikely at the Memorial Tournament in July when the 36-year-old struggled with the putter and carded consecutive rounds of 80 to miss the cut.

"I don't know if he'll talk about it, but I played with him at the Memorial ... and I've never seen him as lost, anywhere remotely close to that lost," Thomas told reporters.

"He's been out here for a while. He knows having a short memory is good out here." – Justin Thomas

"He was putting so bad, playing so bad. But he never gave up. He wasn't quitting."

Johnson withdrew from the 3M Open in Minnesota the following week with a back injury after struggling in the first round.

"He was trying to find it out there and he couldn't again," Thomas added. "He's been out here for a while. He knows having a short memory is good out here.

"And then next thing you know, what, two months later he's the FedExCup champion and running away. That's all you need to know about golf right there."