At the start of this year, the American was competing on the lower-tier Web.com Tour and simply harboured hopes of securing his playing privileges for 2014 on the main U.S. circuit.
Since then, Spieth has stunningly claimed a first PGA Tour victory, earned a spot on the 12-man U.S. Presidents Cup team and this week has a chance to win the Tour Championship plus the overall FedExCup title with its $10 million (£6.2m) bonus.
"I don't think the year has sunk in," Spieth, 20, told reporters at East Lake on Tuesday while preparing for Thursday's opening round in the final FedExCup playoff event.
"I don't think it should until after the Presidents Cup and I have some off-season. But it's been an incredible year. I've been here and there, in Thursday qualifiers to start the year to making the Tour Championship and the Presidents Cup."
Back in January, Spieth had to play in a Thursday qualifier just to get into a Monday qualifier for the Farmers Insurance Open at Torrey Pines where he missed the cut in his first tournament as a professional.
However, after recording successive top-10s on the Web.com Tour, he returned to the PGA Tour in early March for the Puerto Rico Open where he finished second and has not looked back since then.
"It was just perfect timing, a perfect storm really," Spieth smiled. "I played the best golf that I had played all year (in Puerto Rico).
"That's what you have to do to earn (playing) status the hard way. And then Tampa (for the Tampa Bay Championship) the next week getting seventh, and having status out here."
His strong finish in Tampa earned him special temporary member status on the PGA Tour, making him eligible for unlimited sponsor exemptions for the rest of the year.
Spieth went on to win his maiden PGA Tour title at the John Deere Classic in July to secure full membership status before losing out in a playoff for last month's Wyndham Championship.
Earlier this month, he recorded his eighth top-10 of the year with a tie for fourth at the Deutsche Bank Championship, then earned one of two wildcard picks for the Presidents Cup by U.S. captain Fred Couples less than 48 hours later.
"At the beginning of the season, my main goal was to earn my PGA Tour card for the 2014 season," said Spieth, who became the youngest winner on the PGA Tour since 1931 with his playoff victory at the John Deere Classic.
"But it turned out better than I could have expected, and I was able to play the PGA Tour this season. With the win, I have job security for a couple of years, and I guess I'm moving up the world ranking."
The youngest player ever to qualify for the Tour Championship, Spieth comes into this week ranked 13th in the FedExCup standings and has a chance to clinch the overall playoff title by winning the PGA Tour finale.
"I'm very excited," he said. "I know kind of what I would have to do to win the FedExCup. I just have to win, and I know what maybe other people have to do too.
"It's pretty awesome. I didn't think it would be a possibility this year at the beginning of the year, but now that I'm here, it's time to maybe make dig in and make something special happen."