What it means: Kirk began the day two shots off the pace, but he grabbed the top spot with three birdies in his final four holes to post a 4-under 68. Na and Martin joined him momentarily at 10 under, but both fell back with miscues on the 18th hole.
Round of the day: Justin Thomas set a new tournament record with 10 birdies, carding a 7-under 65 to vault into contention. The rookie birdied his first four holes and five of his first six before closing with a stuffed approach shot to No. 18.
Best of the rest: Like Thomas, Chesson Hadley began the day at 1 under but made a big move with a 6-under 66. The reigning Rookie of the Year carded seven birdies against just one bogey, including four back-nine birdies.
Biggest disappointment: With the Stadium Course yielding some low scores, both Na and Kelly suffered from final-pairing nerves. They shot matching rounds of even-par 72, including a closing double bogey from Na, to drop from a share of the lead into a share of fifth.
Main storyline heading into Sunday: Kirk will be the man to watch as he eyes his fourth victory, but this event remains unpredictable. Thirty players will tee off within five shots of the lead, a tournament record, and a group that includes Rory McIlroy at 6 under.
Shot of the day: From 139 yards away, Kirk stuck his approach to within a foot at No. 15. That tap-in sparked a run of three straight birdies on Nos. 15-17.
Quote of the day: “Just because it’s a bigger stage doesn’t mean we’re going to suck all of a sudden.” – Kisner on the success of first-time players this week.