Scottie Scheffler fired the first bogey-free round of his career in majors to move into favouritism for the PGA Championship on Thursday.
Scheffler's three-under 67 was among the pick of the morning starters, who had their rounds delayed by almost two hours, and the 2022 Masters champion has moved to the top of the betting.
Jon Rahm had begun the tournament there and hardened after an opening birdie, only to make a host of mistakes in a six-over 76 which seemingly leaves his hopes of back-to-back majors in tatters.
As for Rory McIlroy, the 2012 and 2014 PGA champion fought back from being three-over through nine holes to card a one-over round of 71 to remain within touching distance of the early pace-setters.
Oak Hill had been expected to provide something of a US Open-like test and so it proved, with thick rough, narrow fairways, firm greens and a premium on long driving all in evidence as the first round developed.
And the longest driver of them all made it count, Bryson DeChambeau carding a four-under 66 to lead Scheffler and Corey Conners by one, with Viktor Hovland two-under having recovered from a slow start.
Eric Cole, a PGA Tour rookie who lost a play-off for the Honda Classic in February, went on to move to the top of the leaderboard with four holes to play, with Dustin Johnson joining Scheffler on three-under after a 67.
Scheffler, narrowly beaten in last year's US Open, scrambled par at his second hole, the 11th, before holing a birdie putt from just inside 15 feet at the par-four 14th.
Eight pars followed but he was rewarded for his patience with another putt of similar length at the short fifth hole, before a precision approach to the eighth saw him tap in for a third birdie and move to the top of the leaderboard.
Rahm dropped five shots in six holes from the 16th through to the third, then missed a four-foot bogey putt to drop two shots at the difficult seventh.
The Spaniard, who four-putted the opening green at the Masters before winning that tournament a month ago, got a shot back at the eighth and saved par at the ninth, but was 10 back of the lead when signing his card.
McIlroy had appeared set to head in the same direction but after chipping in for par at the second, his 11th, was able to tap in for birdie at both the third and fourth holes, missing a good eagle chance at the latter.
Three putts from 35 feet was a sloppy way to hand one of those back but McIlroy holed from six feet at the eighth for another birdie on his way to a one-over 71 which might have been much worse.