Spieth had a seven-shot lead with two holes to play and was on the same score - 18-under par - that only Tiger Woods had ever reached at Augusta National. Out of nowhere, he made a careless double bogey on the 17th hole. Ahead of him on the 18th, former U.S. Open champion Justin Rose poured in a birdie putt.
Just like that, the lead was nearly cut in half.
His gutsy short game saved him in the end. Spieth hit a flop shot from a tight lie to 10 feet and made the par putt for a 2-under 70. That steadied the 21-year-old Texan going into a final round that no longer looks like a runaway.
Spieth was at 16-under 200, breaking by one shot the 54-hole record at the Masters held by Woods in 1997 and Raymond Floyd in 1976.
On a day of charges and endless cheers, Spieth now gets to return and do this all over again.
"We've got a long way to go," Spieth said.
Rose closed with five birdies on his last six holes for a 67, and that birdie on the 18th put him in the final group for the first time in a major.
Phil Mickelson wore a pink shirt in honor of Arnold Palmer because he knew he needed a big charge, and the three-time Masters champion delivered a 67, despite two bogeys on the back nine. Mickelson was five shots behind.
Woods and Rory McIlroy will play together in the final round of a major for the first time - but they are 10 shots behind.
Spieth knew what he was facing even before he started.
Woods, who for three rounds has made everyone forget about that guy who shot 82 in the Phoenix Open earlier this year, ran off three straight birdies early in the round, and he threw a victorious fist pump after a most improbable birdie on the 13th hole. McIlroy made eagle on his second hole, went out in 32 and inched closer to Spieth on the back nine.
Both of them stalled.
McIlroy made bogey on two of the last three holes for a 68. Woods made a bogey from the bunker on the 18th for a 68.
For a short time late in the afternoon, Spieth made the green jacket ceremony seem like a formality.
Four shots ahead of Charley Hoffman, Spieth buried a 10-foot putt on the 12th hole and another birdie from about the same distance at the 13th. He followed a three-putt bogey on the 14th hole by making two more birdies, and his lead was up to seven shots as the trees began casting long shadows.
He looked in total control at what is the most peaceful time of the day at Augusta National.
And then it was shattered. Spieth chipped weakly to the 17th green and three-putted for a double bogey. It was a reminder how quickly comfort can vanish.
The story line should sound familiar - a 21-year-old with a four-shot lead going after his first major at Augusta National. Four years ago, that was McIlroy, who shot 80 in the final round.
Now it's Spieth's turn, and he at least knows what to expect. A year ago, Spieth was tied with Bubba Watson going into the last round and had a two-shot lead with 11 holes to play until Watson rallied to win.
"I think the good thing for him is he's already experienced it once," McIlroy said. "He's played in the final group at the Masters before. It didn't quite happen for him last year, but I think he'll have learned from that experience. I think all that put together, he'll definitely handle it a lot better than I did."
McIlroy all but ruled out his chances of adding the Masters to his collection of majors. Only one player in major championship history has rallied from 10 shots behind on the final day. That was Paul Lawrie at Carnoustie in the 1999 British Open, and Jean Van de Velde is nowhere to be found.
Woods wasn't willing to concede.
He mentioned Greg Norman's collapse in 1996 after the second round, and McIlroy's collapse after he finished up his third round.
"I'm going to have to put together a really special round of golf tomorrow," Woods said. "And you just never know. You never know around this golf course."
Spieth was reminded on one hole - a double bogey at the 17th - how quickly it can change.
He starts the last leg of this dominant week with Rose. Right ahead of them will be Mickelson, one of the most popular figures at Augusta, with Hoffman (71). Woods and McIlroy will be ahead of them.
"There's going to be roars," Spieth said. "Phil is going to have a lot of roars in front. A few groups up I think is Tiger and Rory ... well, you're going to hear something. It's about just throwing those out of my mind, not worrying about it, not caring, setting a goal and being patient with the opportunities that are going to come my way."
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