Merritt matched David Frost's tournament mark set in 1994 and topped the leaderboard at 12-under 130, four strokes in front of defending champion Matt Kuchar and John Merrick.
Spieth started off the low, low scoring at Harbour Town Golf Links, making nine birdies in a bogey-free round after struggling in an opening 74. The 21-year-old Texan was six shots back at 6 under.
Merritt had nine birdies over his final 12 holes, finishing with a 5-foot putt on the course's signature lighthouse 18th.
Merrick had a 65, and Kuchar shot 66.
