"I was patient for the most part and aggressive ... when I had opportunities and game felt really, really good," said Casey, who is seeking to become the first back-to-back winner in the 19 years of the event.
"Looking forward to tomorrow, playing with Dustin is going to be very exciting," said Casey. "Any time I got a chance to go up against world number one I'm excited about that."
Johnson applied the pressure with a 67, including a birdie at the last, and fellow American Jason Kokrak (66) was close behind on 206, thanks in part to a hole-in-one at the par-three 15th.
England's Luke Donald and American Scott Stallings were also in contention after both shot rounds of 70 to finish on 207.
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Casey gave himself some breathing room after picking up three shots on the back nine but ended the round on a downer with a bogey at the last.
Johnson also had five birdies but only one bogey and he put on a show with his birdie at the 18th.
"I had a good lie in the bunker, but it was on an upslope and had to get it over the lip so I had to hit it really high," he said. "It was into the wind, up the hill, only had 130 yards I think.
"I hit a full nine-iron and ... luckily I made really good contact with it and it flew just past the hole."
But the birdie putt was a challenge.
"Even then I had whatever, seven feet, and still had a big swinging downhill putt," Johnson said.
"I mean these greens are really fast but a lot of slope on them. Sometimes you hit them really close you don't have easy birdie putts."
The ace was the second of Kokrak's professional career.
He used an eight-iron on the 212-yard hole.
"It (the ball) carried just far enough and took a nice hop and rolled in there," Kokrak said.
(Reporting by Gene Cherry in Raleigh, North Carolina, editing by Ed Osmond/Peter Rutherford)
