The 28-year-old could not match the pace set by joint-leaders Adam Scott and Kevin Stadler, but was pleased with his effort in windy conditions at The Vines.
“It was very windy out there and I am happy with a 68,” said Casey.
“I struck it nicely and if I can continue to hit the ball the same way I will be happy. The wind can be very confusing, it swirls around and it’s easy to misjudge a few approaches.”
Casey birdied two of his first three holes of the day before recording another four birdies on the back nine.
“I played sensible golf and if I missed a green I contrived to get up and down,” the Ryder Cup star added.
“The greens have a lot of undulations but are rolling nicely.”
Flamboyant Englishman Ian Poulter - decked out fully in red - shot out of the blocks with birdies on three of his first four holes.
A double bogey on the 10th and a bogey on the 15th dropped him back to level par, but the world number 67 fought back with birdies on his final two holes to finish two under.
Defending champion Scott birdied his last three holes to finish eight under, but vowed not to think too far ahead. “It was a great opening round and I certainly had a lot of opportunities and I made some putts,” he said.
“Obviously it’s a long way to go. I’m not getting ahead of myself, it’s a course where you can shoot a low score like I did today, but you can also shoot a high one pretty easily.”
Tied with Scott is world number 256 Stadler, son of American legend Craig Stadler.
“I’m having a good time so far and really enjoyed playing the golf course,” the 26-year-old said.
“It’s a great start to the tournament, I love playing courses like this.”
KJ Choi enjoyed a purple patch midway through his round, the Korean shooting six birdies in seven holes on his way to a 65. “My short irons were very good and putting was also good from inside 25 feet,” he said.
“I felt comfortable and could get some good birdies, the speed (of the greens) is good for me and I’m happy.”
Scottish duo Colin Montgomerie and Scott Drummond remain in the hunt after rounds of 70, tied with a host of others including Gary Emerson, Gary Evans and Barry Lane.
Retief Goosen and US Open champion Michael Campbell could only manage rounds of 72.