Warren Bennett, Gary Evans, Peter Fowler and Peter Lonard share third place at minus-5 67.
Casey started quickly with a birdie at the par-4 10th, his first hole. However, his quick start was halted by heavy rains. Casey and the rest of the afternoon players, as well as some of the morning participants who hadn't finished, were called off the course for a 90-minute rain delay.
When play resumed, Casey carded another birdie at the 12th. He moved to 3-under with a birdie on No. 15, but gave that shot right back at the next.
'I was down the left in the rough, not a great lie, tried to hit a soft 9, made a mistake and fatted it into the bunker short,' said Casey. 'It was a good bunker shot but I missed the putt. It was purely a mental error and a lack of concentration, which was disappointing. There was absolutely no reason for making bogey on 16.'
Casey then birdied three of the next four holes around the turn, with the last coming at the second. The Arizona State graduate grabbed a share of the lead with a birdie at the par-4 fifth.
He took the outright lead with a birdie on his final hole of the day, the ninth.
'I only made one mistake out there so I'm very happy with the way it turned out,' said Casey. 'It was a very good round of golf considering the elements and the break in between.'
Luna exploded out of the gate, birdieing five of his first six holes to leap to the top of the leaderboard. He would cool off with pars on his next two holes, but birdied the ninth for a 6-under 30 on the front nine.
Luna however stumbled on the back nine with a bogey at the 11th when he was unable to get up-and-down from a tough lie in the rough over the green. Luna regained that shot with a birdie on No. 12 before parring in for his 66.
Lonard, playing with defending champion Ernie Els, had a steady round with three birdies on the front side and two on the back en route to a bogey-free 67.
Stephen Allan, John Bickerton, Niclas Fasth, Ian Garbutt, Ricardo Gonzalez, Lucas Parsons, Greg Turner and Steve Webster are tied for seventh place at minus-4 68. Nick Faldo is among a group of nine players one shot further back at 3-under.
Els, who has already won twice this season and was runner-up in Singapore last week, is tied for 24th at 2-under. He got off to a poor start with a bogey at the first. After the rain delay, he recovered that shot with a birdie at the fifth.
Els birdied the 10th, but faltered at the 11th to drop back to even par. The reigning British Open champion then birdied 12 and 13, before dropping a shot at the 14th. He finished with a birdie on 16 and a pair of pars to close out the round.
Greg Norman in his first action of 2003 shot a one-over 73.