After playing one hole in the morning, Na waited out a 71/2-hour rain delay in the tournament plagued by wet conditions for the second consecutive year. Ninety-eight players were unable to finish the round, and 39 of them didn't even begin play.
Jimmy Walker was 6-under with four holes to play when play was suspended for the day because of darkness. Mike Weir, trying to become the first Canadian to win the national championship since Pat Fletcher in 1954, opened with 70.
Vaughn Taylor, of Evans, shot 74.
Na closed his first nine, the back nine at Glen Abbey, with birdies on the par-5 16th and 18th, added birdies on Nos. 1 and 2 and birdied the final five for a front-nine 28, matching the tournament record set by Vijay Singh on the same nine in the second round of his 2004 victory.
"The greens are holding, so you can get aggressive with the irons," Na said. "But off the tee you must put yourself in the fairway. The rough is very thick."
He hustled to finish so that he didn't have to return early today.
"That's all I was hoping for," Na said. "I was just trying to get to the tee real quick and hit. Ended up being we had 10, 15 minutes to spare, but you never know when they're going to call it."
Durant had a hole-in-one on the seventh hole.
"It was like 147 to the hole, which for me was kind of an in-between shot," Durant said. "I hit a nice little smooth 8-iron. It hit about 4 feet past and drew back in."
He also was relieved to finish the round.
"We were running the last couple holes to try to get done," Durant said. "It'll be nice to sleep in."
After the round began on time at 7 a.m., heavy rain forced the suspension of play at 8:38. The rain stopped just before noon and course workers needed more than 4 hours to drain the flooded bunkers and get the clay-based layout in shape to play.
Because of the wet conditions after about 11/2 inches of rain hit the course overnight and in the morning, players were allowed to use preferred lies in the fairways -- just as they did in all four rounds last year.