Casey leads by three over Bubba Watson with Chris Kirk one further back and Hideki Matsuyama and Martin Flores one shot further back again.
When asked after his round how he has handled the comeback Casey said; "In sort of baby steps, I think, which is the most frustrating thing. It's slowly crawling along and making progress, climbing back up the ladder, however you want to phrase it.
"And, yeah, it's very difficult when you've played to a certain level and then you genuinely I had genuinely had no clue how to play to that level at certain stages and you start to contemplate: Well, why am I doing this? Is there much point continuing? Do I really want to play this game professionally?
Luckily I didn't think about that too long. And I sort of dove into the challenge of that, and that sort of got absorbed by the process. And that's the easiest way out of it, because if you sit back and think too much, then it becomes very consuming. And so I felt like I was on the edge of being consumed sometimes."
Consecutive rounds of 66 have given Casey his impressive lead over an elite field.
The talk of round two was the massive turnaround of Rory McIlory who recorded a second round of 78 some 15 shots worse than his brilliant start yesterday when he opened with a round of 63.
Was there a concern with the knee injury that seemed to bother him early in yesterday's round? "The knee's okay," said McIlroy after his round. "It was a little sore this morning on the range because if I just keep hitting balls continuously and just keep torquing it, then it's going to get sore. But out on the course it was fine. Painkillers kicked in. And it didn't like I felt it a little bit, but didn't really bother me too much."
Adam Scott is well enough placed in a share of 10th position at 5 under and although seven from the lead he know he is not out of it. "Yes, I guess I feel like I'm playing better than that," said the world number one after his early morning round. "But I didn't necessarily play better than that today. Everything feels like it's in a good place and it was one of those days where nothing was going that great for me, but I ground it out and got it under par and hopefully won't be too far back going into the weekend."
Marc Leishman improved his chances of a place in the US Open field with a round of 68 to share 10th position with Scott and others, the Victorian needing to force his way into the top 60 in the world by June 9th. A top ten this week would more than likely do just that.
Jason Day has done well to be in 24th position with a second round of 69 which considering his absence from tournament golf is a solid and encouraging start to the week.
Aaron Baddeley and Greg Chalmers are in 48th position and are the only other Australians to make the cut.