Rory McIlroy battled illness and a poor start to remain in contention for a third US PGA title as Bryson DeChambeau set the clubhouse target at Oak Hill.
On a course which has drawn comparisons to Winged Foot, the venue for his US Open triumph in 2020, a slimmed-down DeChambeau still possessed enough power to post six birdies and two bogeys in an opening 66.
That gave DeChambeau a one-shot lead over world number two Scottie Scheffler, Dustin Johnson and Corey Conners, with Viktor Hovland, Ryan Fox, Keegan Bradley and Adam Scott on two under par.
American Eric Cole had reached five under par after 14 holes before play was suspended due to darkness, the result of a delay of almost two hours caused by frost on Thursday morning.
“It’s a fantastic round of golf at Oak Hill,” DeChambeau said. “It’s a prestigious place. Very difficult golf course.
“As I was looking at it throughout the week, I’m like, man, I don’t know how shooting under par is even possible out here on some of the golf holes.
“It’s a different test (to Winged Foot) and a test that I’m willing to take on. If you’re driving it well and hitting your irons well, you can play out here, but it can get pretty nasty pretty quick if you’re not hitting it straight.”
McIlroy fought back from three over par after nine holes to return a 71, while Masters champion and world number one Jon Rahm surprisingly slumped to a six-over-par 76.
Following a delay of almost two hours due to overnight frost, Oak Hill member McIlroy had started from the 10th hole with five straight pars before badly mishitting his tee shot on the 15th to find a bunker 30 yards short of the pin.
McIlroy hit a good bunker shot to nine feet but was unable to convert the par putt and also failed to get up and down from sand on the 17th, before taking a swipe at the tee marker on the 18th following an errant drive.
The four-time major winner looked certain to drop another shot after firing his third to the second over the green, but holed from 35 feet up a steep slope for an unlikely par and then hit a brilliant tee shot to two feet on the next.
“When you walk off the green three over and then you hit a tee shot like that, all of a sudden the pendulum swings or momentum goes the other way and you feel like you’re right back in the tournament.
“Depending on what happens over the next three days and what I go on to do, I may look back at that shot as being the sort of turning point of the week.”
McIlroy reduced the 617-yard fourth hole to a drive and long iron to set up his second birdie of the day and although he three-putted the next, a birdie on the eighth helped complete a battling round despite being under the weather.
“I’m fighting something,” McIlroy confirmed.
“I thought I got a great night’s sleep last night and I looked at my Whoop (personal fitness monitor) and I was 22% recovery, and my skin temperature was 3.5 degrees higher than what it’s been.
“But I actually feel better today than I felt yesterday, so plenty of water and a bit of rest, I’ll be fine.”
Rahm had started his day with a birdie on the 10th, his opening hole, but dropped five shots in six holes around the turn and also made a double bogey on the seventh.
“Obviously the first six holes of the day I played really good,” Rahm said.
“Put myself in a good spot and after that I found myself battling. Couldn’t find the fairway and the fairways that I missed cost me bogeys.”
Play had been scheduled to get under way at 7am local time (1200 BST), but predictions of cold conditions overnight proved accurate and led to tee times being delayed by an hour and 50 minutes.
The possibility of weather delays had been on the cards since the US PGA was moved from August to May in 2019, with Oak Hill having already been selected as this year’s venue.