The South Korean carded a second consecutive round of 68 to add to his course record 63 on day one.

America's Pat Perez is in second place at nine-under with 18 holes to play.

Choi has already won on the PGA Tour this season, taking the Compaq Classic in May.

And for a third successive day he looked on course for a second title in 2002 as his closest rivals faded.

Choi started his round confidently with a birdie on the par-fourth opening hole.

He added four further birdies at the sixth, seventh, 12th and 14th before his only blemish of an otherwise faultless round on the 15th.

In an uncharacteristic blip, the South Korean double bogeyed the hole but, by then, already boasted a six-stroke lead.

Victory would catapult him into a top-20 place on the current money list and all but guarantee his place in the Tour Championship.

Afterwards he said: "I just wanted to start the day off carefully and play my game.

"I birdied the first hole and got a boost from the get-go. I'm disappointed to three-putt for a double bogey, but I shot three under and, all in all, that's a satisfying day."

Perez, who had started the day three strokes behind Choi, came close to matching his rival on course before a double bogey at the 16th.

Perez, though, ruled out his hopes of catching the overnight leader.

He said: "I just gave away too many shots to give myself a real chance."

Glen Day and Donnie Hammond share third place at eight under, while Australian Rod Pampling, in second place overnight, stuttered to a round of 73 to drop to fifth place.