Spieth, in a three-way tie for the lead with world number one Jason Day and Derek Fathauer when the second round concluded early Saturday, pulled inexorably away with a sparkling effort at Pebble Beach Golf Links, one of three courses in use over the first three rounds of the tournament.
The two-time major champion, seeking his first PGA title of 2017, opened with back to back birdies. His only bogey of the round, at the eighth, was followed by three birdies in a row at the ninth, 10th and 11th.
Having picked up another shot at 15, he rolled in a 16-footer for birdie at the par-three 17th, and left himself inches for a birdie at 18.
His 17-under par total of 198 put him six in front of American Brandt Snedeker, who carded a five-under 67 at Pebble Beach.
It was a further stroke back to two-time winner Dustin Johnson and unheralded Kelly Kraft, Americans who posted 65s for 205.
World number one Day made a disastrous start to the third round with three bogeys and a double bogey in his first six holes at Pebble Beach.
He finished on a stronger note with three of his five birdies in his last four holes -- but not before his card was marred by another double bogey at his 14th hole of the day, Pebble's par-three fifth.
Day signed for a three-over 75 that left him 10 off the pace.