The Englishman mixed six birdies with an eagle and two bogeys at the Muirfield Village Golf Club to move into 12 under.
Masters champion Bubba Watson (69) is three shots adrift in outright second, with American countryman Chris Kirk (70) third at eight under.
Northern Irishman Rory McIlroy, who carded a brilliant nine-under 63 to open the tournament, produced a 78 to slip to tied for 24th.
Casey, starting on the back nine, holed birdies on the 11th and 13th holes before an eagle at the par-five 15th.
The first of his blemishes came at the 17th, but he responded with birdies at the 18th and first.
Birdies on the fifth and seventh holes followed, only to bogey the ninth to finish an otherwise strong round poorly.
“I was ecstatic with the way I played,” Casey said.
“There was a couple of mistakes in there but that’s Muirfield Village. It’s a tricky golf course and you don’t need to do a lot wrong to make a mistake.”
Watson’s round was a rollercoaster one as he produced six birdies and an eagle to go with five bogeys.
Japan’s Hideki Matsuyama (67) and American Martin Flores (68) are tied for fourth at seven under, a shot behind Kirk.
Americans Hunter Mahan (70), Ryan Moore (70) and Scott Langley (66) are joined by Dane Thorbjorn Olesen (67) at six under.
World number one Adam Scott (70) remains in contention in a tie for 10th at five under as he looks for back-to-back wins having claimed the Crowne Plaza Invitational on Sunday.
American Matt Kuchar (69) is unlikely to defend his title, sitting tied for 48th at one under.
England’s Justin Rose and American Rickie Fowler were among those to miss the cut.
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