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.
"If we can grind those kinds of rounds out and shoot under par on tough courses, we are in good shape," said the 22-year-old Texan, already a double major champion.
"Just holding in there when I felt like it was all going wrong, to be actually in contention and not have thrown myself out of it, this bodes well. Hopefully we can improve on today."
What also helped was that Spieth stopped talking to his caddie after every single poor shot, a bad habit he had developed earlier this season.
"I've been getting overly frustrated ... I'm the one hitting the shots but I keep on talking about why I didn't hit it where I wanted to hit it and I've just been keeping that to myself now, at least I did today," he said.
"And today was a great day to test it. Really I think we did a great job mentally, my best mental round I've had in a long time."
(Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Larry Fine)
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