Im Sung-jae leads The American Express by one shot after the second round, while Phil Mickelson made history on the PGA Tour despite missing the cut.
South Korean golfer Im carded a seven-under-par 65 on the PGA West Stadium Course to hit the front in California on Friday.
Im – who finished tied for second at last year's rescheduled Masters, and is eyeing his second Tour title after breaking through at the 2020 Honda Classic – was flawless with seven birdies and no bogeys.
The 22-year-old is 11 under at the halfway stage of the tournament, ahead of Nick Taylor (66), Tony Finau (66), Abraham Ancer (65), Kim Si-woo (68) and overnight leader Brandon Hagy (70).
Defending champion Andrew Landy posted a second-round 69 to be tied for 37th and six strokes off the pace.
In his 2,201st career PGA Tour round, five-time major champion Mickelson carded 18 consecutive pars for the first time.
Mickelson – playing for the first time since November's Masters – following his opening-round 74 with a 72 as the American star finished seven shots outside the cut line.
"For that to be the first time, it's really shocking,” said Mickelson. "Surprising, because I really try to hit fairways and centers of the greens and just make easy pars.
"Looking at the scores, you're going to say, 'Gosh, you played two-over par, pretty easy courses, pretty poor start'. But I feel like there were a lot of good things to take from the week, even though the scorecard’s not going to show it."