Tense back-nine fight underway for Masters crown
Jordan Spieth (C) of the US reacts after making a birdie putt from a bunker on the fourth green during the final round of the 78th Masters Golf Tournament at Augusta National Golf Club on April 13, 2014 in Augusta, Georgia - by Emmanuel Dunand
A four-shot swing over the last two holes on the front nine in Watson's favor left him in the lead at the turn on eight under with Spieth on six under after a bogey-bogey end to the front side.
Sweden's Jonas Blixt was third behind the Americans at four under and Matt Kuchar was fourth on three under as they began the 10th hole.
Watson curled in a tricky 12-foot birdie putt at the ninth to stand on eight-under par at the turn while Spieth lipped out on a five-foot par putt at nine for bogey to stand on six-under.
Spieth missed a five-foot par putt at the par-5 eighth while Watson tapped in for a birdie that had put both of them on seven under.
Spieth, a 20-year-old Texan, could become the first golfer to win the Masters on his first try since Fuzzy Zoeller in 1979.
The poised US prodigy could also become the youngest major winner since Tom Creavy took the 1931 PGA Championship.
In addition to the green jacket symbolic of Masters supremacy, the champion takes home $1.62 million from a record $9 million purse. Second place pays $972,000.
Spieth, who began the round level with Watson on five under, was two strokes ahead on the fourth tee thanks to a birdie at the par-5 second, a bogey by Watson at the third and a four-putt double-bogey by Kuchar, who briefly shared the lead after birdies at the second and third.
After finding a bunker at the par-3 fourth short of the green, Spieth blasted out of the sand and rolled the ball in the cup for a shocking birdie to reach seven under and elicit a roar from the spectators.
Watson followed with a birdie to stay two strokes back but Spieth's dazzling shot seemed to reinforce his date with destiny.
Yet Spieth's lead was trimmed after his first bogey of the day at the par-4 fifth. Spieth and Watson each then birdied the par-3 sixth.
When Spieth followed with a four-foot birdie at the par-3 sixth and a 10-foot birdie at the par-4 seventh, he reached eight-under, two ahead of Watson.
Spieth could continue a unique historic tradition at Augusta National.
Jack Nicklaus became the youngest Masters champion in 1963, replaced by the late Spanish superstar Seve Ballesteros 17 years later in 1980.
In 1997, 17 years after that, it was Tiger Woods who became the youngest Masters winner ever.
And now it's 17 years later again with Spieth -- who is 20 years, eight months and 16 days old -- poised to nudge under Woods's youth age mark by seven months.
Spieth is the first Masters rookie in Sunday's final duo since 1995 and only the third since 1937.
Blixt, like Spieth, could win the Masters in his first try and could also be the first Swedish man to claim a major crown as well as Europe's first Masters winner since Spain's Jose Maria Olazabal in 1999.
World number one Woods, absent after surgery to ease a pinched nerve, appears set to keep his top ranking next week with those able to overtake him -- 2013 Masters champion Adam Scott, his fellow Australian Jason Day and Sweden's Henrik Stenson -- floundering well back.
There have been 15 first-time major winners in the past 19 majors and 19 different winners in the prior 21 majors, only Rory McIlroy and Phil Mickelson winning twice in that span.