HILTON HEAD ISLAND, 17 Apr 2015 (AFP) - Merritt's 12-under-par total of 130 put him four strokes ahead of defending champion Matt Kuchar, who posted a 66, and John Merrick, who signed for a 65.
"I had my tempo down the stretch, especially with the irons," said Merritt, whose 10 birdies included seven on the back nine -- four in a row to end his round with a bang.
"It was fun to see the putts roll in," added Merritt, who matched the course record set by South African David Frost in 1994.
"I plan to enjoy the moment, hit some good shots, not think about it too much and just add them up at the end of the day," he said of his approach to the weekend.
Spieth helped get Friday's birdie bonanza going at Harbour Town, South Carolina with nine birdies in his nine-under effort.
The round vaulted him into contention after an opening 74 in which he looked to be feeling a let-down after capturing his first major title at Augusta National on Sunday.
"I found something in my ball position on the range this morning and it made a significant difference in my ball-striking and in my putting," the 21-year-old Texan said.
He walked off the course with a share of third on a leaderboard topped by Kuchar, before Merritt roared to the top.
Jim Furyk, the 2010 champion, carded a seven-under 64 to join a group sharing fourth place on 135, alongside 2013 winner Graeme McDowell (69) and Kevin Kisner (67).
Spieth finished the day in a group on 136 that also included South Africa's Louis Oosthuizen and Americans Morgan Hoffmann, Matt Every and Kevin Streelman.
He said his disappointing opening round motivated him "a lot," as did playing alongside Kuchar.
"The hole looked bigger out there with the putts he was making and I was just trying to keep up," Spieth said.
Kuchar finished the day with six birdies and one bogey and enjoyed his close-up view of golf's newest sensation.
"He's playing fantastic golf," Kuchar said of Spieth. "I was surprised to see (Thursday) not go as well for him... so I was happy to see that he got it rolling."
Leading second-round scores on Friday in the $5.9 million US PGA Tour RBC Heritage (USA unless noted):
130 - Troy Merritt 69-61
134 - Matt Kuchar 68-66, John Merrick 69-65
135 - Graeme McDowell (NIR) 66-69, Kevin Kisner 68-67, Jim Furyk 71-64
136 - Morgan Hoffmann 68-68, Matt Every 66-70, Louis Oosthuizen (RSA) 69-67, Jordan Spieth 74-62, Kevin Streelman 71-65
137 - Jerry Kelly 71-66, Justin Thomas 70-67, Bo Van Pelt 69-68, Lucas Glover 70-67, Stewart Cink 70-67, Aaron Baddeley (AUS) 70-67, Sean O'Hair 70-67, Blake Adams 72-65, Branden Grace 70-67, John Peterson 72-65
138 - Scott Langley 68-70, Jim Renner 69-69, Vijay Singh (FIJ) 71-67, Bryce Molder 74-64, Ben Martin 69-69, Brice Garnett 72-66, Brendon de Jonge (ZIM) 70-68