The Norwegian star signed for a sizzling eight-under 64 around a wet Albany Golf Club on Saturday, enough to take him to 13 under for the tournament, three ahead of the chasing Scottie Scheffler.
The defending champion set the back nine alight with a red-hot putter, rolling in six birdies before a mud-ball on the 18th led to a bogey, giving him a chance at becoming the only player other than Tiger Woods to win the tournament back-to-back.
"I hit a lot of just good quality iron shots to give myself seven to 12 feet," the 25-year-old told pgatour.com.
"It wasn't like I hit one just incredible shot and stuff it or make it like I did the other days, but it was just kind of consistently giving myself looks."
The parts of the rain-soaked course - including the fairways - made for challenging conditions, as some players were forced to deal with mud on their balls. Justin Thomas, who's six under 66 moved him into T3 on eight under, luckily managed to avoid the worst of the mud.
"It's unfortunate," the two-time major winner said of the course conditions.
"And you can get some really, really unlucky breaks and unfortunate situations. But like I said, everybody has to deal with it and the more you let it get to you, probably the worse off you are."
Veteran Kevin Kisner set the gallery around the 12th green into raptures when he had a hole-in-one with his 6-iron, but had to settle for a disappointing 77.