Sweden's Stenson, boosted by two eagles on the front nine, was atop the leaderboard by two strokes at the 18th tee, only to take a quadruple-bogey 8
on the closing hole and settle for a one-under par 71.
"It was a very disappointing finish," Stenson said. "But I had good fun out there. I had two eagles and I got it going. I didn't hit a fairway after 11. I hit it all over the place."
Stenson still has not cracked 70 in 19 rounds at Augusta National.
"If you can't get the ball in play off the tee you're going to drop shots," Stenson said.
"Playing out of the forest most of the back nine, it's going to cost a little bit sooner or later -- disappointing that it cost that much."
Stenson's eight matched the worst score ever recorded at Augusta National's closing hole, the seventh time it had been shot there and the first since Colombia's Camilo Villegas did it in 2007.
Just last year, Stenson took an 8 at the fourth for the worst score in Masters history at that par-3 hole.
"I didn't expect it to cost as much as it did on 18," Stenson said. "That's normally what happens. You make a little mistake and then you compound it with another one and it just keeps on snowballing and I got the snowman in the end."
Stenson eagled the par-5 third and seventh holes with 15-foot putts and birdied the par-4 fourth as well. After starting the back nine with a birdie, he went bogey-birdie on 14 and 15 but had a two-shot cushion at the par-4 18th.
On 18, Stenson hooked his tee shot left into trees and chipped out into a squishy footprint lie.
"It was like in a pretty worn spot in the pine needles where everybody has walked," Stenson said.
"If I would have gotten the second shot out on the fairway, it would have been a different story."
He hit into the right rough, went over the green into the green with his fourth shot, then chipped to the fringe.
"The worst part of 18 was really over-hitting the wedge and hitting it up in the crowds behind the green and then it took me four to get down from there," Stenson said.
The Swede ran his downhill putt past the cup, then missed a three-footer coming back before tapping in to end the misery.