Nick O'Hern birdied each of his last two holes to join Peter Fowler and Paul Lawrie in a tie for second at 10-under-par 203. Marten Olander was one shot further back at 9-under-par 204.
Casey was one of several players who had to return to the course early Saturday morning to complete the second round. He managed to take a four-shot lead after 36 holes and held on to that lead in what turned out to be a very long day.
The Englishman parred each of his first two holes before picking up a birdie at the fourth. Casey then struggled off the tee at the fifth en route to a bogey and left his second shot short of the green at the sixth.
Casey could not get up and down for his second dropped shot of the day. The 26-year-old buckled down, however, and landed his tee shot inside seven feet for a birdie at the par-3 ninth.
He played his tee shot to the fringe at the par-3 11th but managed to roll in the long birdie putt to reach 15-under.
Casey hit a bad drive at the 12th and the result of his recovery shot was even worse. Casey, who was trying to hook his second shot, slammed his ball into a tree and watched as it vaulted in the opposite direction of the green.
He bogeyed the hole and was in trouble again at the 13th but nearly saved par. Casey scrambled short of the putting surface but his putt for par from off the green almost found the bottom of the cup.
Casey recovered with a birdie at the 15th and, with darkness approaching, he missed a chance to extend his lead at the last by a couple of inches.
Andrew Coltart, Stephen Gallacher, Ricardo Gonzalez and Maarten Lafeber shared sixth place at 8-under-par 205.
Sergio Garcia carded his second straight round of 71 to finish seven shots off the lead. Garcia was joined by Padraig Harrington, Angel Cabrera, Pierre Fulke and Robert Karlsson in a tie for 10th at 7-under-par 206.
Amateur Gonzalo Fernandez-Castano finished alongside Ignacio Garrido, Gregory Havret, Miguel Angel Martin and Jarrod Moseley at 6-under-par 207.