The American world No 2 produced dismal form from tee to green, especially on his front nine where he hit just one green in regulation, but he "did a great job mentally" as he made par putt after par putt to work his way into contention.
Spieth's fortunes improved markedly after the turn and he reached six of nine greens in regulation before capping a round to remember with birdies on his last four holes at Firestone Country Club in Akron.
"I really won the battle in my head today," a smiling Spieth told Golf Channel after totalling just 21 putts and chipping in to save par at the eighth.
"I could have shot four or five-over pretty easily with the way that I was striking the ball tee to green.
"We (he and his caddie, Michael Greller) have really tried to approach this tournament as the turnaround for the second half of the season, and I think it was a really good start."
Renowned for his brilliant putting, Spieth took great encouragement from the way he had been able to eke out a sub-par total on a challenging layout while not at his best.
