Spieth sank a 41-foot birdie putt from just off the green on the par-three 16th to reach 14-under-par 202 in the final PGA Tour tuneup before the first major of the season. He's coming off a title and a runner-up finish in his previous two starts, having climbed to No. 4 in the World Golf Rankings.
A victory Sunday would allow Spieth to wrest the PGA point standings lead away from Jimmy Walker and further boost his confidence at Augusta National, where he came in second to Bubba Watson last spring.
Austin Cook, who got in through the Monday qualifier, is tied for second place at 203 with first-round leader Scott Piercy and Johnson Wagner, the tournament's 2008 champion.
The 24-year-old Cook is still seeking his PGA Tour card. He finished the second round Friday with four consecutive birdies to catch Phil Mickelson for second place, then briefly took the lead with a birdie on the eighth hole Saturday.
He had an up-and-down back nine, suffering a double bogey on the par-five 15th, but reclaimed a stroke with a birdie on 17 and calmly sank a short putt for par to stay on Spieth's heels.
Piercy had tied the tournament course record with an opening 63 but skied to 74 Friday, only to bounce back strongly with a third-round 66. Wagner posted a 67 to fight his way into contention.
Andrew Putman, the 36-hole leader, struggled to a 75, falling five shots off the surging Spieth's pace. Mickelson, who had left the course as the leader Friday, birdied the first hole before coming apart. The three-time Masters champion also had to settle for a 75, leaving him four strokes back.