The 20-year-old American carded a three-under-par 67 on the long and challenging East Lake Golf Club layout, two birdies in his last seven holes leaving him five strokes off the lead, alone in third place.
"I felt like I shot as low as I could have shot, which is kind of a rare case this year with me," Spieth told reporters after posting a five-under total of 135 in the PGA Tour's season finale where FedExCup playoff honours are also on the line.
"With my ball-striking, I've given myself a lot of opportunities in the past couple of months and my good tournaments were the tournaments where I felt comfortable putting.
"Today, the ball-striking was a little off, but I must have had probably 25 putts. It wasn't that many. I felt really good over the ball and I'm excited where I'm at."
Spieth trails Sweden's Henrik Stenson by five shots heading into the weekend with only Masters champion Adam Scott, at six-sunder, between them.
"Henrik's obviously tearing it up, so we have some work to do," Spieth said of the Swede, who holds a four-stroke advantage over Australian Scott.
"But if I can go for the greens (in regulation), kind of get that feeling down tomorrow and strike it a little better, it could be a low round."
Spieth has enjoyed a fairytale debut campaign on the PGA Tour, starting the year without any status and ending his season by qualifying for the elite field of 30 players at East Lake.
He 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 bonus.
"I'm just so still stoked about being at this tournament, what it means," Spieth beamed. "And then the Presidents Cup is in two weeks, and that obviously keeps you going.
"More than twice the amount (of golf) I've ever played in a year, and it's only September," added the American, who has recorded eight top-10s in 22 starts on the 2013 PGA Tour.
The youngest player ever to qualify for the Tour Championship, Spieth began this week ranked 13th in the FedExCup standings but has ruled out his slim chance of clinching the overall playoff title by winning the PGA Tour finale.
"From this point with Henrik first and Adam tied for second, it's kind of not the scenario that would be perfect for me," said Spieth, who won his maiden PGA Tour title at the John Deere Classic in July to secure full membership status.
"Yeah, I don't expect to win the FedExCup at this point, but I can make a run at the Tour Championship."
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