Watney carded a bogey-free seven-under-par 64 to join Na, who fired a 66, at the top of the pile after round three - with the US duo holding a one-shot lead over the field.
The closest challengers are Robert Garrigus, who equalled the best round of the day with a 63, and Kris Blanks (66) on 16-under-par, with a group of three players - Tommy Gainey, Paul Goydos and Tim Herron a further shot back on -15.
This is Watney's seventh event in nine weeks and after a disappointing end to the FedEx Cup, the 30-year-old is looking to bounce back in Las Vegas. He admits he has changed his strategy to try and succeed.
"I've been trying to stay a little more even keel this week, whereas during the play-offs I would probably get a little more frustrated, and it didn't help anything," he said.