South Korea's Im Sung-jae, a Masters runner-up last year, fired a bogey-free seven-under par 65 to grab a one-stroke lead after Friday's second round of the US PGA American Express tournament.
The 22-year-old Asian star stood on 11-under 133 after 36 holes at PGA West in La Quinta, California, in an event using the par-72 Nicklaus and Stadium courses.
"I feel like my mid-range putts were really doing well," Im said. "I dropped a lot of mid-range putts and that's what led me to a good score today."
South Korean Kim Si-woo, Canada's Nick Taylor, Mexico's Abraham Ancer and American Brandon Hagy shared second on 134 with Italy's Francesco Molinari, American Doug Ghim and Argentina's Emiliano Grillo another stroke adrift.
Im, who began on the back nine at the Stadium course, sank a four-foot birdie putt at the par-4 12th, a 10-footer to birdie the par-3 13th and rolled in a 25-foot birdie putt at the par-4 18th.
After opening his back nine with a par at the first, Im sank a 20-foot birdie putt at the par-4 third, a 21-foot birdie putt at the par-4 seventh and a 17-foot birdie putt at the par-5 eighth.
"I was struggling with my putting leading up to this week, so I worked on my putting a lot and especially for today's round," Im said.
"I started drawing a line on the ball and as I visualize my putts and that seemed to work pretty well today."
Im, the 2019 PGA Rookie of the Year, won his first tour title at last March's Honda Classic just before a three-month Covid-19 shutdown and finished second to Dustin Johnson in his Masters debut last November.
After that, Im stayed at his new home in Atlanta instead of going to South Korea and facing mandatory isolation.
Kim, the 2017 Players Championship winner, birdied four of his first six back-nine holes at the Nicklaus layout but the 25-year-old from Seoul needed three strokes to reach the green at the par-3 17th and made bogey to fall one back of Im as he chases a third PGA victory.
World number 398 Hagy, seeking his first PGA title, only made the field when second-ranked Spaniard Jon Rahm withdrew. The back nine Stadium course starter could have shared the lead as well but followed birdies at seven and eight with a closing bogey.
Taylor needed only 25 putts on the Stadium course. He birdied the second and the par-5 fifth and eighth holes and three in a row starting with a 23-foot putt at the par-4 15th followed by a tap-in at the par-5 16th and an 18-foot birdie putt at the par-3 17th.
"I didn't make a whole lot of putts until 15," Taylor said. "Just to see the ball go in was nice and I made a great one on 17 too... most of the day I was scaring the hole."
"I drove it really nice. My iron play was solid. I was chipping pretty nice and when I got out of position I was able to make pars."
Finau fired a 66 on the Nicklaus course to share second despite trouble off the tee.
"Got away with a couple (poor) drives. I was able to turn those bad drives into good scores and that let me turn the round around," Finau said.
Molinari, a late contender in the 2019 Masters, had four birdies on both the front and back nine but a double bogey at the ninth on the Nicklaus course kept him from sharing the lead.
"The wind switched and threw me under the bus," Molinari said. "Looking forward to the weekend."
Four-time major winner Phil Mickelson made 18 pars to shoot 72 and miss the cut on 146, the first time in 2,201 career PGA rounds he parred every hole.
"For that to be the first time, it's really shocking," Mickelson said.
Leading scores after Friday's second round of the US PGA American Express tournament at La Quinta, California (par-72, USA unless noted):
133 - Im Sung-jae (KOR) 68-65
134 - Nick Taylor (CAN) 68-66, Tony Finau 68-66, Abraham Ancer (MEX) 69-65, Kim Si-woo (KOR) 66-68, Brandon Hagy 64-70
135 - Emiliano Grillo (ARG) 69-66, Francesco Molinari (ITA) 69-66, Doug Ghim 67-68
136 - John Huh 68-68, Brendan Steele 68-68, Max Homa 66-70, Andrew Putnam 67-69, Charley Hoffman 70-66, Roger Sloan (CAN) 69-67, Brian Harman 68-68, Adam Schenk 68-68
137 - Rory Sabbatini (SVK) 68-69, Ryan Armour 70-67, James Hahn 68-69, Luke List 69-68, Sam Ryder 67-70, Josh Teater 68-69, Paul Casey (ENG) 72-65, Richy Werenski 69-68