"It wasn't playing easy out there," Na, 31, told PGA Tour Radio. "The wind was a bit breezy and it was tough to judge the distance on some of the shots.
"But I hung in there today and broke par. I've still got the lead and I'm looking forward to tomorrow."
Englishman Poulter, seeking his third win on the U.S. circuit, missed a birdie attempt from seven feet at the par-four last and had to settle for second place after returning a 68.
Charley Hoffman carded a 66 to finish at nine under, a stroke better than fellow Americans Chris Kirk (65) and Brandt Snedeker (66).
Na, whose only PGA Tour victory came at the 2011 Justin Timberlake Shriners Hospitals for Children Open, offset a birdie at the third with a bogey at the fifth to reach the turn in level-par 35.
He picked up another shot at the par-five 11th before succumbing to a two-stroke swing at the par-four 15th where he bogeyed after missing a five-footer for par.
Playing partner Poulter, who is known for his prowess on the greens, drained a 32-foot birdie putt at the 15th to draw level with Na before the American regained the outright lead at the 17th with his third birdie of the day.
"I trusted my read, a little outside right and it turned nicely into the hole," Na said. "It was nice because I was under par going into the last hole."
Australian Adam Scott, who won last year's tournament in a playoff, was five strokes off the pace after shooting a 66, ending the day level with Masters champion Jordan Spieth (67).
Due to threatening weather, the players set off earlier than scheduled in threesomes with a two-tee start.
Similar conditions have been forecast for Sunday with players again teeing off early in threesomes, from 7 a.m. CT (1200 GMT), in a bid to finish before expected thunderstorms.
(Writing by Mark Lamport-Stokes in Los Angeles; Editing by Gene Cherry)
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