Na had a 10-under 130 total for a two-stroke lead over Ian Poulter.
Na had 26 bogey-free holes in a row until his tee shot at No. 18 went into a rain-swollen drainage culvert and was swept away by flowing water. He two-putted from 20 feet after his approach to the back side of the green for what he called a "good bogey."
Poulter had a 67. Boo Weekley was third at 7 under after a 69.
Masters champion Jordan Spieth was alone in the lead before losing four strokes in a two-hole span. He finished with a 73, nine strokes worse than his opening round that had him in a four-way tie on top. He was seven strokes back at 3 under.
