Im leads Fleetwood by one at Quail Hollow
Im birdied the par-five 10th and 15th holes and answered a bogey at the 12th hole with a birdie at 14 to stand on nine-under 133 after 36 holes at Quail Hollow in Charlotte, North Carolina.
"I'm very happy I'm leading," Im said. "I didn't putt very well, but I'm happy with where I'm at.
"I'm driving the ball very well. I'm keeping it in the fairways, so it makes it easier for me to hit my second shots on the green. I know this course very well, so I'm taking advantage of that."
Fleetwood fired a 67 to finish one adrift with England's Alex Fitzpatrick and American Justin Thomas sharing third on 135 after each fired 68.
Rory McIlroy, coming off a triumph in last month's Masters, shot 67 despite a bogey at 18 to join a pack in eighth on 137 that included Americans Rickie Fowler, who fired a 63, and Matt McCarty, the first-round leader.
"Would have been nice not to make that bogey at the last," McIlroy said. "But overall it was a good day's work and puts me right into the thick of things going into the weekend."
Im, a 2020 Masters runner-up, seeks his third PGA Tour title after the 2000 Honda Classic and 2021 Shriners Children's Open.
The 28-year-old Asian star had struggled with a wrist injury but said he was "100%" this week.
Fleetwood sank a 13-foot birdie putt at 14 and made a five-footer for birdie at the par-three 17th to grab second.
"Definitely didn't have my rhythm, got ahead of a few shots, but I didn't hit it in any terrible spots and I scored very well," Fleetwood said.
"I've definitely hit enough good shots. Today I didn't quite have it, but still managed to get a good score out of it. I holed out well and did a lot of other things well."
McCarty, who fired a 63 on Thursday, shot 74 with three bogeys and a double bogey.
McIlroy back in form
World number two McIlroy, who had played only the Masters over a seven-week span, found his form as he charged into the hunt for his fifth victory at Quail Hollow.
The Northern Ireland star sank a 17-foot birdie putt at par-five 11th, made an 13-foot birdie putt at 11 and dropped his approach inside three feet then tapped in for birdie at the par-three 13th.
McIlroy reached the green in two and tapped in for birdie at the par-five 15th then made an 11-foot birdie putt at 16 to pull within three of the lead before finding fairway and greenside bunkers at 18 to set up a bogey.
A day earlier he sank a 15-foot birdie putt at 18 after 17 pars.
"I still feel like I left a couple out there," McIlroy said. "It was an improvement and if I can keep seeing improvements as the week goes on, that's a good thing.
"There's maybe a little rust, but it's more like getting a (scorecard) in your hand and getting comfortable hitting certain shots. Definitely felt a little more comfortable today, drove it in the fairway a little bit more and was able to take advantage of that and make some more birdies."
American Chandler Blanchet aced the 188-yard, par-three 17th hole.