Though he was caught in a share of the lead by both Jason Kokrak and Robert Garrigus earlier in the round, the 23-year-old forged ahead with three birdies in the last six holes to post a 15-under total of 198.
"You've just got to approach it the same way," Koepka, who is competing for the first time in a regular PGA Tour event, told Golf Channel of his mindset for Sunday's final round.
"We've got a good game plan and things seems to be going pretty well. So just stick to it and everything will take care of itself. I played really well today."
The big-hitting Kokrak carded a 68 to share second place at 13 under with fellow American George McNeill, who rocketed up the leaderboard with a 10-birdie 62, equalling the course record matched earlier in the day by compatriot Jimmy Walker.
"It was a lot of fun," McNeill, who is seeking a third victory on the PGA Tour, told reporters. "Sixty-two, I don't know how it could ever not be fun.
"I figured out something on the range last night and my swing was much more comfortable today. I felt free so I could go ahead and aim at some flags."
'SENIOR CITIZEN'
Former world number one and 'senior citizen' Vijay Singh, who made his debut on the over-50 Champions Tour in Hawaii last month, carded a 65 to sit three shots off the pace at 12 under, level with Walker.
But it was Koepka, a non-member of the PGA Tour who is playing this week on a sponsor exemption, who once again commanded the spotlight.
The former three-time All-American at Florida State birdied three of the first four holes to remain a stroke in front, Kokrak having briefly joined him in a tie at the top after holing out from 130 yards to eagle the par-four first.
Though Koepka bogeyed the sixth, he parred the next three holes to reach the turn in a share of the lead at 13 under with Garrigus, who mixed five birdies with a lone bogey on the front nine.
Koepka offset a birdie at the 13th with a bogey at the 14th, where he found a fairway bunker off the tee, but regained a one-shot cushion when he got up and down from a greenside bunker to birdie the par-five 15th.
He picked up another shot at the driveable par-four 17th, two-putting there from 30 feet, and narrowly missed out on a birdie at the last where his eight-foot putt stayed out.
Koepka, who has already won four times on the lower-tier European Challenge Tour, is bidding to become the first person to triumph on the PGA Tour playing on a sponsor exemption since England's Lee Westwood at the 2010 FedEx St. Jude Classic.
"The Challenge Tour, winning four times, that helped me a lot me in learning how to win," said Koepka.
"That's a big thing. You've just got to stick with what you're doing for a whole week. That's what got you here in the first place."
(Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Greg Stutchbury)
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