Casey, who had started the day one off the lead following an opening 66 on Thursday, stayed in touch at the top of the leaderboard on seven under following a spectacular fightback.
The 39-year-old world number 14's roller coaster round included two bogeys and a triple-bogey eight against six birdies.
"Not every day you enjoy a round of golf with an eight on the card, but I'm a pretty happy man," Casey said. "It was a bit of a roller coaster."
Casey started confidently, bagging a birdie on the 11th to move to seven under alongside Fowler.
But his round started to unravel on the 437-yard par-four 12th when he bogeyed after finding the fairway rough.
It got worse on the par-five eighth, when a triple bogey sent him plunging down the leaderboard.
A further bogey on the next hole seemed to spell the end of his round.
However Casey showed superb resilience to come roaring back, stitching together a run of five consecutive birdies.
He drained a 12-foot birdie putt on the 17th and then tapped in from seven inches for another birdie on 18 after a delicate chip on.
A magnificent iron to two feet set up a birdie putt on the long par-five first following the turn before an 11-foot birdie putt on the next hole took him back to six under.
Yet another solid approach gave him a chance for a 10-foot birdie on the third, which he sank.
Casey is now relishing the prospect of being in the mix at the top of the leaderboard over the weekend.
"There will probably be more nerves, more excitement, there is a lot of golf to be played here," he said.
"I've shown what can happen with one bad swing. So you've got to be very, very patient and stay in the moment. The last thing you need to be doing is thinking ahead, especially on this golf course."
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