Former world number one Tiger Woods, who has won this event four times, is tied for second with fellow American Steve Stricker on three under.

Woods birdied four of his first five holes and two more on the back nine but three bogeys checked his progress.

England's Paul Casey is last in the elite 18-man field on seven over par.

Casey birdied the first hole but a triple-bogey eight on the par-five second followed and six more shots were dropped in his final six holes as he finished with a seven-over-par 79.

Scotland's Martin Laird also struggled in the windy conditions at Sherwood Country Club in California, bogeying eight holes as he recorded a five-over 77.

Laird was playing alongside Choi and their rounds could not have been more different.

Choi opened with a quintet of birdies and sank another on the 13th to move to six under.

His only dropped shot came on the par-three 15th when he took three to get down from the fringe of the green but he immediately atoned for that with a birdie at the par-five 16th.

Woods also opened brightly, racing to four under par after five holes. A dropped shot on the ninth was regained on the 13th but consecutive bogeys on the 15th and 16th threatened to derail his round.

However, he drained a 15-foot putt on the 17th and two-putted for par on the last to remain firmly in contention.

Woods, without a win in two years, was joined on three under by world number six Stricker, the highest-ranked player in the field, who holed a 25-foot birdie putt on the last.

America's Matt Kuchar was handily placed at four under par on the 16th tee, but he walked off the green with a triple-bogey eight after getting tangled in the rough.