Oakmont reared its nasty head as Dustin Johnson made a topsy-turvy start to his third round on Saturday.
Heavy downpours on day one made the greens somewhat easier to play on Friday but Johnson's brief wobble on his front nine was a stark reminder of the challenges of the notoriously difficult course.
A double-bogey at the par-four third hole saw overnight leader Johnson - who had only carded one bogey for the entire tournament up until that point - drop into a three-way tie for the lead alongside Lee Westwood and Andrew Landry in a stunning swing of momentum.
Seeking his maiden major title, Johnson bounced back immediately with a birdie next up but his attempted par save at the sixth was unsuccessful as he fell to three under, alongside Sergio Garcia, Westwood and Landry, as Shane Lowry moved a shot clear.
Riding the wave of his runner-up finish at April's Masters, former world number one Westwood had earned a share of the lead with an eagle at the fifth hole - his second hole-out in Pennsylvania - having birdied the second, but he dropped the seventh to slip a stroke behind, only to jump back up the leaderboard.
World number one Jason Day, meanwhile, is continuing to climb at even par, though defending champion Jordan Spieth is four shots worse off.