Emiliano Grillo, Beau Hossler, Andrew Putnam and Jhonattan Vegas were all tied for third at six-under.
Steve Stricker, who appeared to injure his back on the fourth hole, recovered nicely to shoot a 65.
Na's remarkable chip came after a heated exchange with long-time caddie Kenny Harms on the previous shot on the par-four ninth.
Na and Harms clashed over which club to use on his approach and Na ended up missing badly as the ball sailed over the green and settled up against the grandstand.
After being granted relief from officials, Na hit his lengthy chip perfectly for a career highlight and the outright lead.
"That chip was almost impossible and I don't know how it went in but I'll take it," he told Golf Channel.
Harms said he knew the two clubs Na was considering for the approach shot would put him over the green and while that proved correct, he felt the 34-year-old was vindicated by his spectacular chip.
"I said it three or four times, 'no, no, I don't like it'," Harms said.
"But he makes the ultimate decision and it turns out he was right again, it was the right club."
Na said Harms was right about the approach in hindsight but was happy with the final outcome.
"It worked out great," he said. "I got a good break."
(Reporting by Rory Carroll; Editing by Ian Ransom)
