Casey was at 8-under 136 after two rounds. Defending champion Miguel Angel Jimenez (70), David Horsey (71), Anthony Wall (71) and Soren Kjeldsen (69) were next.
Colin Montgomerie and Rory McIlroy were in a group at 2 under. Jose Maria Olazabal, voted into the World Hall of Fame this week, birdied his last two holes for a 70 and joined John Daly (71) at 144.
Course designer Ernie Els birdied the last for a second 73 that left him right on the cut of 2 over, in a group that included Graeme McDowell, who shot 71 despite shin splints in his right leg. An MRI scan revealed McDowell did not have a stress fracture, which could have forced him out for several weeks.
Those missing the cut included Masters champion Angel Cabrera (75), the 2005 winner, and world No. 4 Henrik Stenson (70), who needed an eagle at the last and could make only par. Lee Westwood was out after a second successive 77.
Casey said his round of two eagles, four birdies, and three bogeys was a bit frustrating.
My ball-striking was better today and I hit a lot of good shots at the flags but I still made a few too many mistakes, the Englishman said. If I look at the number at the bottom, Im pleased, but not with the last five holes.
That span included both bogeys, one at the 17th where he drove into trees.
By contrast, Casey started well, with an eagle at the long 4th.
I think my drive hit a sprinkler head or a downslope because it went miles. Then I hit a 7-iron to seven feet, Casey said.
His second eagle, at the par-4 13th, came on a 9-iron 163 yards that bounced once and dropped into the hole for a two.
Casey has won Abu Dhabi and the Houston Open this year, and was runner-up at the Accenture Match Play. He has risen from 41st at the start of the year to seventh in the world rankings, and has not reset his goals.
I wrote them down at the start of the year and they were fairly lofty. I havent had to redo them, he said.
Jimenez had two birdies and two bogeys on his first seven holes, and followed that with 10 straight pars. He finished with a 25-foot putt for eagle on his last hole.
He drove into a fairway bunker on 18 but a perfect 5-wood put him on the front of the green.
My game today was not as good as yesterday but I played decent, the Spaniard said. A 70 was about the right score for how I played today.
Daly atoned for a poor day with his irons by chipping in three times on the back nine, including once from a bunker.
On a perfect day with little wind, scoring was still not easy on the fast-running course redesigned over the past two years by Els. The pin placements were also tough.
Ernie has done a great job, Daly said. He has really made you think where you have to hit it with some of the bunkers.