Koepka, ranked 33rd in the world, carded a five-under-par 66 for a total of 15-under-par 269, one better than Masters champion Bubba Watson (65), 22-year-old Japanese Hideki Matsuyama (67) and American Ryan Palmer (66).
"It feels amazing," said Floridian Koepka. "The last few weeks I put in a lot of hard work, changed my putting stroke completely and everything seems to be going right."
Scotsman Martin Laird, the 54-hole leader, was tied with Koepka until he bogeyed the driveable par-four 17th with his first three-putt after going 95 consecutive holes without one.
Laird then found water with his drive at 18 and took a double-bogey to register a 72 for 12-under, a total shared by Spanish amateur and Arizona State student Jon Rahm, who closed with a 68.
The self-assured Koepka, who claimed his first European Tour victory at the Turkish Airlines event in November after winning four times on the European Challenge Tour, began the round three shots behind Laird.
It was Matsuyama who made the first charge.
Matsuyama, who was three back, got off to a flying start when he holed out from a sand-filled divot in the fairway 129 yards from the cup for an eagle-two at the first hole to reach 12-under.
Birdies at the third and fifth lifted Matsuyama, who won last year's Memorial Tournament, into a tie with Laird at 14-under, and a birdie at the par-five 13th gave the Japanese a one-shot lead.
One hole later there was a three-way tie for first at 14-under and another hole later it was four-way tie with Palmer joining Matsuyama, Laird and Watson.
Then Koepka rolled in his eagle putt to vault into first at 15-under, a total matched by Laird with a birdie at the same hole before the Scotsman unravelled at the end.
Koepka has now shot par or better in all 12 rounds on the tour this season.
(Reporting by Larry Fine in New York; Editing by Gene Cherry)
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