In the same ruthless manner in which he won a second US Open title this year, Clark romped to a five-shot third-round lead at the BMW Championship in Missouri on Saturday.
The 32-year-old American backed up successive rounds of 64 with a 65 to surge to 17 under for the tournament, with world No.2 Rory McIlroy among his closest pursuers.
It is the second time this year Clark has led by at least five shots going into the final round. He had a six-shot advantage at the US Open and held on to win by one despite hostility and heckling from the New York crowd at Shinnecock Hills.
He is going for his third win this year, which would put him in the conversation for PGA Tour player of the year with his major championship.
"You start seeing the lines and you're so much more committed" Clark said of his putting. "You're like, 'OK, that's exactly it.' And then you just stroke it and the ball goes in."
McIlroy moved into contention with his own 65 to join Patrick Cantlay (66) at 12 under, with fellow Americans Chris Gotterup (68) and Gary Woodland (70) one shot further back.
"That's a lot to make up," McIlroy said of his deficit. "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."
Clark stumbled with a double-bogey six on the first hole to lose a share of the halfway lead to Woodland but roared back with six birdies and an eagle in a seven-under 11-hole mid-round burst.
Scott is a distant 14 strokes adrift of Clark at three under, in a tie for 28th in the 50-man field in St Louis, after only managing a third-round 71.
The 46-year-old Scott briefly slipped outside the Tour Championship qualification bubble with four bogeys in his first 10 holes on Saturday (Sunday AEST).
But Australia's former world No.1 steadied the ship with priceless birdies on the 13th and 17th holes to only drop two spots to 29th in the season-long standings.
Only the top 30 after Sunday's final round will progress to next week's $US40 million season-ending showpiece in Atlanta.
But another decent round keeping him in the top 35 fior the tournament should be enough to advance - 20 years after winning the Tour Championship.
Fellow Australian Min Woo Lee is already assured of making his first Tour Championship, despite sitting in equal 45th at one over following a third-round 70.
Lee is 21st in the points race after starting the week comfortably in 18th position.
