Two months after the 21-year-old wowed the golfing world by winning the Masters at Augusta National, Spieth has once again returned to the winner's enclosure having secured a one-shot victory over fellow American Johnson and South Africa's Louis Oosthuizen at Chambers Bay last night.

The Texan from Dallas becomes the youngest US Open champion since the great Bobby Jones in 1923 and is the first golfer since Tiger Woods in 2002 to complete a Masters-US Open double in the same season.

Spieth, now halfway to a Grand Slam of four majors in a year, had started the final round with a share of the lead at four-under-par with Johnson, Branden Grace of South Africa and Australia's Jason Day.

He even fell behind after bogeying the par-four first and then saw Johnson open up a two-shot lead at the turn.

But such was the see-saw nature of a dramatic final day on Puget Sound that the championship was not decided until the last man played his final shot at the 72nd hole.

Gradually, the contenders faded. Day, left exhausted by the after effects of his bouts of Benign Positional Vertigo on Friday and Saturday, finally ran out of steam on the final day, carding a four-over 74 to finish on level-par for the tournament.

Johnson appeared to fade with bogeys in four holes at the start of his back nine, leaving Grace and Spieth in an apparent head to head.

Even Rory McIlroy had got in on the act for a short time, clambering onto the leaderboard at two-under and three shots off the pace early in the leaders' rounds.

Fellow Irishman Shane Lowry failed to get his final round going having started three off the overnight lead.

Lowry would rally on the back nine before bogeying the last for a 71 to finish on level par with Day and McIlroy, who carded a 66, in a tie for ninth.

Grace went to the 16th tee level with Spieth at four under only to drive out of bounds, his tee shot clearing the boundary fence, stopped only by a second chain-linked fence separating the course from a railway line.

He double bogeyed and the error was compounded when Spieth birdied the same hole, the three-shot swing handing the young American what seemed like the tournament on a plate, three clear with two to play.

But this drama was not played out. Spieth unexpectedly folded on 17, missing the par three's green and double-bogeying himself to fall to four-under, just as Louis Oosthuizen reached the same mark with a birdie at the 18th, his sixth in the final seven holes to sign for a 67.

And still the twists kept coming. Spieth nailed his second shot on 18 to 16 feet and two-putted from there for birdie to edge clear of the South African at five-under with a closing 69.

And right on cue, Johnson striped his drive 353 yards down the 18th and knocked his approach shot to 13 feet to leave him a 12ft 4in eagle putt for victory as Spieth looked on helplessly from the recorders hut.

It rolled past the hole by some four feet but Johnson still had a chance to force a play-off with his birdie putt. He missed it and the man who had thrown away two chances of major victory in 2010 at the US Open and PGA Championship had blown another, his three-putt handing victory to Spieth.

“I'm in shock,” Spieth said. “It's hard right now. It's hard. I'm still amazed that I won, let alone that we weren't playing tomorrow (in a play-off).

“So for that turnaround right there, to watch that happen, I feel for Dustin, but I haven't been able to put anything in perspective yet.”

Spieth did acknowledge the assistance he got from caddie Michael Greller, who as a local-area teacher had started his caddying career at Chambers Bay when the course opened in 2007.

“I think it will sink in a little quicker than the Masters did given that it's already happened but, boy, what a team effort. What a team effort the whole week.

“I didn't have my best stuff and we were able to get it done. Michael knew this course better than anybody playing this week and he made sure I was in the right spots without my best stuff and that's why I won.”

Former Masters champion Adam Scott shot the round of the day, a six-under 64, to sneak into a share of fourth place with fellow Australian Cameron Smith (68), one stroke behind Oosthuizen and Johnson on three-under.

Another Augusta National winner, Charl Schwartzel, also finished strongly with the South African's 66 lifting him into seventh place at two-under, American Brandt Snedeker the last man home under par, by a shot, after a 68.

Spieth had walked up 18 in a tie with Johnson coming up behind in the final group and had a big decision to make over the eagle putt in front of him, go for broke or play safe and two-putt for birdie.

“Walking up to the green, I was saying you got to be careful. But I felt like I needed eagle. I needed to eagle. I told myself you need to eagle to tie.

“Major champions don't leave these putts short. That's what I was telling myself. I hit a decent putt. I didn't leave it short. It got to the hole. It had die speed, which was the speed I wanted. I misread it by a smidge.”

Yet it proved enough for Spieth to become US Open champion.