South Korea's Im Sung-jae birdied three of the last four holes but top-ranked Dustin Johnson clung to a one-stroke lead after Saturday's second round of the US PGA Tour Championship.
Im fired a six-under par 64 to finish on 12-under but struggling Johnson, last in the field in driving accuracy after finding only 2-of-14 fairways, closed with a four-foot birdie putt to lead on 13-under after 36 holes at East Lake in Atlanta.
"Tough one. It was up and down. Just didn't drive it straight enough," said Johnson. "Everything else feels good. I managed my game pretty well.
"I couldn't really attack the golf course being in the rough and in the bunkers. I think I was everywhere today."
Im, 22, would be the second-youngest FedEx Cup winner, two months older than Jordan Spieth when the American captured the 2015 crown.
Johnson scrambled to a 70 to hold off charging Im with American Xander Schauffele third on 11-under after a 65 and third-ranked American Justin Thomas fourth on 10-under.
"The rough is thick and nastier than I've ever seen it," Schauffele said.
Spain's Jon Rahm, PGA Championship winner Collin Morikawa and England's Tyrrell Hatton shared fifth on 9-under in the season-ending showdown for a $15 million playoff prize.
Johnson matched his fewest fairways hit in a PGA round, managing only two also at the 2009 and 2011 Phoenix Open.
"I need to drive it better over the weekend. Frustrated with my tee balls," Johnson said. "If I want to win this golf tournament, I've got to hit more fairways. Just poor execution. trying to guide it down the fairways instead of swinging it."
Johnson began the week on 10-under by virtue of his season points lead with Rahm next on 8-under, third-ranked American Justin Thomas on 7-under and other rivals up to 10 strokes adrift in the staggered start system.
World number 27 Im, last season's US PGA Rookie of the Year, began the event six strokes behind Johnson but charged with his third round of 65 of lower since the June PGA restart after the COVID-19 pandemic.
Rahm and Johnson began day two level for the lead at 13-under and the American was solid early.
Johnson rolled in a 37-foot birdie putt at the third hole to seize the lead at 14-under, then sent his tee shot at four deep into the left rough only to blast out, wedge onto the green and sink a 31-foot par-saving putt.
Rahm dropped his approach inside 4 1/2 feet at the fifth and made the birdie putt to briefly share the lead, but Johnson sank a six-foot birdie putt at the par-5 sixth to reclaim the lead alone.
Johnson's roller-coaster finish to the front nine saw him miss the seventh fairway on the way to bogey, then blast out of a fairway bunker to 43 feet and hole the birdie effort only to find a bunker at the par-3 ninth and take another bogey.
Johnson sprayed a tee shot deep in the left rough at 12, blasted from pine straw to deep greenside rough and made bogey. Johnson had an 11-foot birdie putt at 14 roll around the edge and horseshoe out of the hole, settling for par to stay in front.
Rahm found water off the tee at the par-3 15th and made double bogey then took another bogey at 16 to fall back.
Johnson found a bunker at 15 and missed a 12-footer for par and, after Im had matched him for the lead, birdied the last to grab the lead.
- Im-pressive showing -
After a run of three birdies and a bogey in four holes on the front side, Im opened the back nine with a five-foot birdie putt at the 10th.
Im dropped his approach inside eight feet at the par-3 15th and made the birdie putt, then put his approach at 16 inside three feet to set up another birdie to reach 11-under.
By the time he reached the par-5 18th green in two, Im was looking at a 30-foot eagle putt to seize the lead alone. He rolled the putt three feet past the cup but made the birdie putt to briefly share the lead with Johnson.
Defending playoff champion McIlroy, who became a father Monday with the birth of daughter Poppy, fired a 71 in quest of his third US PGA playoff crown in five seasons.
McIlroy, who shared eighth on eight-under with American Brendon Todd, has not managed a top-10 finish since the US PGA Tour returned from a three-month COVID-19 shutdown in June.