But then McIlroy blew up with double-bogeys on three holes in a row in a 78 that left him at 3-under 141.
"I saw the numbers before I went out to play," Casey said. "But I have no idea how that happened."
McIlroy had a good idea how it happened, however.
"I missed fairways. That was the big thing," he said. "I didn't realize how thick the rough was until I got into it today."
Even on his round through the first three holes, his second shot caught tree branches on the 13th hole and kicked deeper into a thicket as he made a double on the par-4 hole. At the 14th, his approach came up short and landed in the creek in front of and to the right of the hole. Then at the 15th, his second shot ended up in the thick rough left of the par-5 hole and he double-hit the wedge shot for a one-shot penalty on his way to a 7.
"But it's not disastrous," McIlroy said. "Even though I had such a bad day, I'm still in it. So, going into the weekend, not exactly where I want to be — but it could be worse."
Watson, who won his second Masters last month, got to within a shot of Casey before bogeying the final two holes in a 69 that kept him three shots back. That was where he was at the start of the day, only now he's chasing Casey instead of McIlroy.
Chris Kirk shot a 70 and was alone in third at 136, with Hideki Matsuyama, who gained knowledge of the course from playing in a losing cause at last fall's Presidents Cup, shot a 67 and was at 137 along with Martin Flores (68). Hunter Mahan (70), Ryan Moore (70), Scott Langley (66) and Thorbjorn Olesen (67) were another shot back at 6-under.
"I can't look at the bogeys; I've got to look at where I'm at," Watson said. "If you told me it's my best two days around this golf course, I'd take it."
Casey said his down years hadn't given him amnesia when it came to knowing how to win.
"It's a bit like riding a bicycle," he said. "I've done it."
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