Jim Furyk is alone in second place at 4-under-par 68, followed by defending champion Retief Goosen, Tim Clark and Luke Donald, who are knotted in third at minus-3.
Ernie Els, a three-time winner of this event, made his return to competitive golf on Thursday after a four-month layoff. The three-time major champion has been out of action since July with a knee injury and the rust was apparent in round one.
The South African managed an even-par 72 and is part of a group tied for seventh place with Chris DiMarco and Adam Scott.
Els started out flat with a bogey at the first, but mixed a birdie and a bogey over the rest of his front nine. He got back to even-par with a 40-foot birdie putt at the 15th and even reached 1 under par with a birdie at 17. Els hit a poor chip at the closing hole and left with a bogey to finish with his 72.
'It was the good, the bad and the ugly,' said Els, whose last competitive round came at this year's British Open. 'I hit some good shots, but it was an awkward round. I'm not despondent. If I get everything together, I can shoot low tomorrow.'
Clarke collected his first birdie at the par-4 third hole, then recorded back-to-back birdies from the eighth to close his front nine in 33. He parred his first two holes on the second nine, then birdied 13 and 14 to move one clear of Furyk.
Angel Cabrera is alone in sixth place at 1-under-par 71. Following the Els group at even-par is American Stewart Cink, alone in 10th place at 1-over-par 73.
Two-time winner Sergio Garcia struggled on Thursday with a 2-over-par 74 and is 11th at plus-2. Kenny Perry rounded out the 12-man field with a 4-over 76.