Spieth on course to win Masters
At just 20 years old, Spieth would replace 21-year-old Tiger Woods as the youngest title winner at Augusta National, as well as becoming the first player since Fuzzy Zoeller in 1979 to win on his tournament debut.
And the American star looked completely unfazed by the occasion as he birdied four of his first seven holes to claim a two-shot lead over former champion Bubba Watson.
Spieth, who only turned professional in December 2012, hit poor tee shots on the opening two holes but made par on the first and then holed from 15 feet for birdie on the par-five second.
The par-three fourth hole had already cost England's Lee Westwood a potentially fatal double bogey and also saw title rival Matt Kuchar four-putt for a double bogey, but Spieth had no such worries as he holed out from a greenside bunker for an unlikely birdie.
Playing partner Watson had bogeyed the third and was briefly three behind, but the 2012 champion then calmly holed from five feet for his own birdie to close the gap.
Spieth was unable to conjure up more magic from a greenside bunker on the fifth, but a brilliant tee shot to the sixth set up another short birdie putt which he converted after Watson had holed from 10 feet for a two.
Another birdie on the seventh took Spieth to eight under par, two clear of Watson with Kuchar another two strokes behind.
Sweden's Jonas Blixt was another shot back on three under with Denmark's Thomas Bjorn two under and 50-year-old Spaniard Miguel Angel Jimenez, aiming to become the oldest winner of any major, another stroke back.
Westwood, seeking a first major title at the 64th attempt, began the final round just three off the lead but dropped three shots in his first four holes before a birdie on the ninth took him to level par.
Earlier in the day, Holland's Joost Luiten finished his final round just as the leaders set off, but the Ryder Cup hopeful's superb 67 suggested an exciting finale was in store.
Luiten birdied four of the last five holes to finish four over par on his tournament debut, taking advantage of some more generous pin positions than in Saturday's third round.
"It feels really good to shoot 67 on Sunday at the Masters," said Luiten, who made the halfway cut on the mark of four over before a third round of 77.
"I look at my tournament and I'm very happy. Yesterday was a little bit of an off day, and in the future you need four good days to compete here.
"It felt like they watered the greens a little bit overnight. They were a little bit slower and a little bit more forgiving than yesterday when they were very slick and quick.
"So it felt like they set it up a little bit easier, but there were still a lot of tough pin positions and you just have to play smart to some of them and take your par and then go on to the next."
Spieth's two-shot lead turned into a two-shot deficit in the space of just 20 minutes, a three-putt bogey on the eighth followed by another dropped shot on the ninth when his approach span back off the green.
Watson took full advantage, getting up and down for birdie on the eighth and holing from 12 feet on the ninth to head into the famous back nine at eight under par.
Spieth was six under and Blixt four under, with Kuchar also dropping a shot on the ninth to be three under.