Spieth was three under par after 14 holes of his opening round at Carnoustie before running up a double bogey on the 15th, dropping another shot on the 16th and scrambling for a bogey on the 18th after driving into the Barry Burn.
The resulting 72 left Spieth six shots behind clubhouse leader and housemate Kevin Kisner as he seeks to emulate Ireland's Padraig Harrington in 2008 in holding on to the Claret Jug.
"The problem [on 15] was on the second shot," Spieth said. "I should have hit enough club to reach the front of the green, and even if it goes 20 yards over the green, it's an easy up and down.
"That's what I would consider as a significant advantage for me, recognising where the misses are, and I just had a brain fart and I missed it into the only pot bunker where I could actually get in trouble, and it plugged deep into it.
"It was a really, really poor decision on the second shot and that cost me."
Spieth was among the morning starters who looked to have got the best of the conditions and the 24-year-old added: "Yeah, it felt like a missed opportunity. I felt like I was really going well.
"I've done a bit of that this year, just the decision making that's cost me. But the misses towards the end were the exact misses that I've been having and I'm working away from. My swing just didn't quite hold up to the end of my round.
"I think I'm certainly in a recoverable situation. I imagine this is as easy as the course could play so I don't see the winning score being any better than it was in 2007 [seven under par} with tomorrow's forecasted tough conditions and Sunday's forecasted heavy winds.
"A solid round tomorrow to stay in the top 25, top 20 will be kind of the goal tomorrow to feel like I can do something on the weekend."