Shane Lowry bogeyed the 18th as he settled for a share of The Open Championship lead at Royal Portrush.
The Irishman, who is seeking a first major title, looked set to displace J.B. Holmes at the summit as he stood over a birdie putt at the last with a one-stroke lead.
However, he three-putted from there to drop a shot and slip to eight under par, alongside Holmes.
Dylan Frittelli - the last man to qualify for the event - looked on course to at least match that score, but toiled to a double-bogey six at 17.
Lowry birdied four of the first five holes, making the most of favourable weather early in his round, and had held firm amid worsening conditions until the final green.
Holmes had earlier carded a 68 to back-up his 66 from Thursday, with Tommy Fleetwood and Lee Westwood one shot further back.
Joining that English duo in the chasing pack was compatriot Justin Rose, the 2013 U.S. Open champion signing for a 67 at a tournament where he shared second place last year.
The seemingly ever-present threat of Brooks Koepka at majors looms large heading into the weekend, the form man on five under.
But some lesser lights edged ahead of the world number one at the midway point, with Cameron Smith and Justin Harding, who shot a 65, firmly in the mix.
It was a day that signalled the end of the road for Tiger Woods, though, the Masters champion's one-under-par 70 not enough to mitigate his woeful opening-round 78.
Home hope Rory McIlroy seemed destined to suffer the same fate despite impressively clawing back four shots over his first 11 holes to get to four over.
Reigning champion Francesco Molinari was able to kick-start a revival, though, getting back to the projected cut mark of one over at the turn.