The American world number one, who tied for fourth here last year, has always loved the iconic Riviera layout but he had never before played it in rain-softened conditions.
And he paid a heavy price for not being able to adjust.
Renowned for his brilliant putting, Spieth missed eight putts from inside 10 feet as he mixed eight bogeys and a double with just two birdies to card his worst opening round on the PGA Tour.
Only twice before on the U.S. circuit, with an 82 at the 2013 Memorial tournament and an 80 at the 2014 Tour Championship, has he fared worse, and he ended the day a distant 16 strokes behind pacesetting Colombian Camilo Villegas.
"It's just a day to forget," the Masters and U.S. Open champion told reporters. "It's one in, hopefully, every couple years. Just unfortunate when it actually does."
Spieth said the horror round came out of the blue.
"It's amazing, I feel really confident about where my game is and had a great range session. There was nothing pinpointed that was wrong."
The 22-year-old has already won seven times on the PGA Tour, most recently at the Hyundai Tournament of Champions in January.
He has established a reputation for a superb short game and mental toughness but on Thursday he was left shaking his head after hitting several shots that did not match his expectations.
"I was just very surprised when I looked up on some of the iron shots," the Texan said. "I felt like I put good swings on them and I looked up and all of a sudden, they were further off-line.
"Like the one on 18 ... I look up and I think that the ball is going to be fading toward the hole the way I strike it, and it's not," he said of his approach at the par-four last that missed the green to the left from where he made a double.
"I didn't expect things to go poorly. I actually expected something to go in. But then when you start getting these five, six-footers on softer poa annua (grass), that's where I really struggle. It certainly showed today."
For all his struggles at Riviera, though, Spieth struck a positive note before he headed off to the range.
"Again, it's just a day to forget, and I won't think much on it," he said. "I'm not throwing this tournament away. I believe that I can shoot 10-under-par on this golf course.
"There's an eight-under round right now (from Villegas), and I think I can do that. I'm not packing it in by any means."
(Editing by Peter Rutherford)
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