Tournament host Tiger Woods fired a five- stroke lead after two rounds of the Chevron World Challenge.
Woods finished 36 holes at eight-under-par 136. Woods, who has won this event four times, is looking for his first win anywhere since the 2009 BMW Championship.
Woods and Choi played together on Friday and battled for the lead most of the round. Woods rolled in a four-footer for eagle on No. 2 and came right back with a birdie on the third to join Choi at minus-six.
Choi responded with birdies on four and six to regain his three-stroke lead as Woods stumbled to a three-putt bogey at the sixth.
After Woods rolled in a 12-footer for birdie at eight, Choi failed to save par at the ninth and his lead was down to one.
Woods caught fire around the turn. He poured in a long eagle effort at the par-five 11th to join Choi at eight-under. Choi sank a four-footer for birdie on the same hole.
At the 12th, Woods sank a nine-foot birdie try and came right back with a birdie on No. 13. He came up short on the par-five 13th, then nearly holed his chip, but had to settle for a tap-in birdie.
"This was sweet," Woods said on TV of his chip on 13. "I thought it would lip in on right edge."
Choi got up and down for birdie on 13 to remain one back.
The par-three 15th proved to be the big turning point. Woods dumped his tee shot in the water short of the green. Choi did the same.
Woods pitched to five feet, but two-putted for double-bogey. He still picked up two strokes on Choi.
Choi's third shot also fell short into the water. He eventually got up and down for quadruple-bogey to tumble to minus-four.
Both Woods and Choi rebounded with birdies on the par-five 16th. Woods was four clear, but he tripped to a bogey at 17 to give him a three-stroke lead after 36 holes.
"That was probably the highest I could have shot today," Woods said in a televised interview. "I hit the ball well all day, but missed a few short ones."
Choi parred the last two to share second.
Kuchar birdied the first, but gave that stroke back with a bogey at the third. He bounced back with birdies on four and five. He had three birdies in a five- hole stretch from the ninth.
After a birdie at 16, Kuchar stumbled to a bogey at the last to end at minus- five.
"I played well again today, but walked off with a bogey at 18. All in all, I'm excited where I stand," Kuchar said in a televised interview.
Rickie Fowler carded a two-under 70 and is alone in sixth at three-under-par 141. Jason Day (68) is one stroke back at minus-two and Gary Woodland (70) is the final player in red figures at one-under-par 143.
NOTES: Although it offers world-ranking points, this is an unofficial PGA Tour event...Jim Furyk, the only other former winner in the field, tripped to a 74 to slide into a share of ninth at plus-one.