England's Ian Poulter, who started at the 10th hole, also got hot in his second nine with four birdies in a 67 for second place at 132, one better than 2013 winner Boo Weekley.
Weekley overcame a nightmare start of four bogeys in his first five holes to shoot a 69 for 133.
Na rolled in a 25-footer to birdie the 10th, hit a 20-foot birdie at 13 that moved him to do a celebratory 'Big Bird dance' and a left-to-right curler for birdie at 14.
"I had a lot of fun today," said Na. "I was making a lot of putts, felt a lot of support and that put a smile on my face."
After splashing his tee shot in the creek at 18, he dropped across the hazard and hit a brilliant 8-iron out of the rough and two-putted to save bogey.
"Happy to walk away with a five," he said.
Morning rain further softened Colonial and more rain was forecast for the weekend, but that did not bother Poulter.
"It doesn't always rain in England, but it can," said Poulter. "It's just keeping your grips dry and keeping as many gloves as you can in the bag and obviously taking your time to hit your shots."
Weekley battled back after a bad start.
"I struggled right out of the gate," said Weekley. "A little bit of rain and my back wasn't loosened up yet, but finally got loosened.
"To be able to finish with four bogeys and shoot one under par for your round, that's real good."
Masters champion Jordan Spieth, an overnight co-leader with Na, Weekley and Japan's Ryo Ishikawa, reached eight-under with a birdie on his 10th hole before two bogeys and a triple-bogey late in his round gave him a 73 for 137, seven strokes behind Na.
"Five-over on the last six holes was very disappointing," Spieth said. "Just a couple of bad swings was all it was."
Ryo shot a 74 to be two-under at 138.
(Reporting by Larry Fine in New York; Editing by Gene Cherry)
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