The American carded an adventurous five-under 65 around Bellerive on Saturday, enough to take him to 17 under for the tournament, five strokes ahead of the chasing Rory McIlroy and Patrick Cantlay.
Clark had a disastrous start to his third round. He pulled his tee shot into gnarly rough, hit his second into the right rough, came up short of the green with his third and missed a seven-footer putt to eventually card a double bogey.
But he rallied. Clark made a couple of gains before the par-five eighth, where he crushed a 5-wood to 12 feet and drained the putt for an eagle. A red-hot putter saw him further extend his lead.
"You start seeing the lines and you're so much more committed," said Clark of his putting.
"You're like, 'OK, that's exactly it.' And then you just stroke it and the ball goes in.'
Meanwhile, McIlroy was solid for his 65 with six birdies and a bogey keeping him in touch. The Northern Irishman admits the gap to Clark is significant, but you never know when you go low.
"That's a lot to make up," confirmed McIlroy.
"But if I focus on myself, try to shoot a good score - I shot a 64 and a 65 this week - shoot something similar tomorrow, you never know."
Similarly, Cantlay isn't giving up the ghost.
"I feel like my game's in a good spot and ready to go shoot a low one tomorrow," he said.
Further down the leaderboard, Gary Woodland and Chris Gotterupp are in T4 for 11 under, while Collin Morikawa is alone in sixth on ten under.