The American young gun sank a curling 28-footer to birdie the 16th and forge three ahead, then shrugged off a double-bogey at the 17th before becoming just the sixth player to slip into a green jacket and hoist the U.S. Open trophy in the same year.

Though Spieth birdied the par-five last to close with a one-under-par 69, he then had to watch as fellow American Dustin Johnson, in the final pairing, had a 12-foot eagle putt to win it at the 18th.

Johnson's putt slid past the cup and he missed a four-foot birdie putt coming back to squander the chance of forcing an 18-hole playoff on Monday at Chambers Bay.

The 21-year-old Spieth posted a five-under total of 275 on a challenging links-style layout that firmed up under a baking sun as he became the youngest winner of the U.S. Open since Bobby Jones in 1923.

"I'm in shock," Spieth told broadcaster FOX. "Wow. I watched it with (caddie) Michael (Greller) in there.

"I just wanted a fighting chance tomorrow. I feel bad for Dustin. I had that feeling on 17. I'm just proud of the way we rebounded on 18."

Asked how he had rebounded from his double at the par-three 17th where he pushed his tee shot way right, Spieth replied: "I was happy 18 was a par-five first and foremost, and I put a drive right where I wanted to.

"I got a nice little bounce off the side and two-putts later ... I didn't think it was good enough but I couldn't be more happy right now."

Spieth, who stunned the golf world with a wire-to-wire victory by four shots at the Masters in April, became the youngest player to win back-to-back majors since Gene Sarazen in 1922 (U.S. Open and PGA Championship).

Johnson, seeking his first major crown, had to settle for a share of second place with South African Louis Oosthuizen, who sensationally birdied six of the last seven holes, including five in a row from the 12th, for a 67.

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