Spieth agreed he should not have played the shot and his caddy had tried to call him off the attempt.
"I had a gap where it had to rise over a tree then under another branch and split (a gap) maybe a quarter of this billboard," he said pointing at the press conference screen.
Spieth felt his four iron fitted the necessary trajectory and matched the distance to the green which was guarded by water on the left. He did not have many other options and a pitch, even if that succeeded, would leave him at least 100 yards short of the green.
The 24-year-old told his caddy to trust him.
"I hit it a touch fat but I knew as long as I got the split it would be okay. It almost hit a ridge (on the green) and went in the water."
It had been extremely special to stay bogey-free on a tough day and the way he was playing.
"I enjoy this tournament more than anywhere else," Spieth said. "It is easy for us, we don't have many distractions in our preparation."
Spieth had to change his Titleist driver the day before the tournament when he cracked the face in practice. It was fine when he used it while he used his three-wood on a number of other holes.
"I got a lot out of the round with what I felt like was kind of average-ish ball striking," Spieth said. "I just scored the ball extremely well, which is something I've been struggling with this season."
"That was a flawless round of golf," playing partner Paul Casey said of Spieth. "When he got into trouble, he bailed out in the right place, and what could have been an error, he turned into a wonderful par save. It was absolutely flawless."
Spieth saved par on at least four tricky occasions while he hit eight fairways, 12 greens and had only 25 putts. If he straightened out his iron play he thought he could be in business at the end of the tournament.
Eight of his nine tournament rounds have been under par with his worst score an even par 72.
