Leading by a record-equaling five shots at the halfway stage of the year’s first major, Spieth had already felt the heat from the likes of Rory McIlroy and Tiger Woods even before he teed off.
Then Phil Mickelson entered the fray with a triple-birdie blast early on that hauled the three-time former winner into firm contention.
But they all needed Spieth to falter and, despite a scare or two early on, he held firm.
A slippery five-footer brought a birdie at the second, but he misread his short putt at the fourth to record just his second bogey of the tournament — against 16 birdies.
A superb 22-footer for birdie at the par-three sixth was then followed by a poor bunker shot at the next that handed him a second bogey.
Hardly the kind of play that stunned the galleries and his rivals in his superb opening rounds, but at that stage he was still where he was when he started the day — five shots ahead of the field.
Spieth was aiming to convert two days of inspired golf into a first major title at the tender age of 21.
The Texan has set Augusta National alight with two rounds of 64 and 66 that saw him record 15 birdies against just one bogey.
In so doing, he became the youngest player to lead the Masters after the first round, and his 130 halfway total is the lowest-ever in 79 editions of the year’s first major.
He was firmly in a winning position, but remained aware that he still had much to do, as he learned last year when he led the Masters after 54 holes, but ended up tied for second behind Bubba Watson.
Spieth’s playing partner, Charley Hoffman, doggedly stayed at nine under with a run of pars before finally sinking a long putt at the eighth for a birdie.
Mickelson joined him there, four shots off the lead, when he hit his approach shot to two feet at the ninth to go out in 32.
That cut the gap on Spieth to four as he approached the turn, but the Texan again responded with a birdie at the ninth to stay five ahead at 15 under.
Dustin Johnson, who grabbed an unprecedented three eagles on Friday, was at eight under and then it was McIlroy and Woods, both of whom were picking their way up the leaderboard after front nines of 32.
The two biggest names in golf were both facing a mountain to climb with 12 strokes separating them from Spieth at the start of the day.
To date, eight strokes has been the biggest halfway deficit anyone has overcome to win the Masters — a feat achieved by Jack Burke in 1956.
McIlroy had already seen his hopes of becoming just the sixth player to win all four Grand Slam titles all but blown out of the water due to two, for him, mediocre rounds of 71.
The 25-year-old Northern Irishman eagled the second and got it to eight under after 15 holes, but dropped shots at 16 and 18 for a 68 that left him well off the lead at six under par.
Woods was upbeat after his 69 on Friday, which saw him play his best golf of what has so far been a wretched year.
Few believed though he could get himself into contention to win a 15th major and fifth Masters, but even a top-15 finish would give him a platform upon which he could continue to rebuild his flagging career.
Once again the conditions were there for good scoring with overnight rain having again softened the usually deadly-fast, undulating greens of Augusta National.
England’s Ian Poulter, who set the early clubhouse lead at four under after a 67, agreed that there were good scores to be had on the course.
“I think there was a few more accessible pin locations today after the overnight rain,” he said. “Definitely softened the course.
“I think you’ll see some of the guys having a chase around that front nine and obviously we know what can happen on the back nine. The holes like 15 are obviously key chances there.”