Shane Lowry saw his Open Championship lead cut to three as Tommy Fleetwood closed the gap on a day when high winds and heavy rain are forecast at Royal Portrush.
The Irishman began Sunday's round - brought forward amid fears over the weather - with a four-stroke lead over Fleetwood following his stunning 63 on Saturday, but he dropped a shot at the first to sit on 15 under.
It is the second time Lowry has taken such a margin into the final 18 holes of a major, having done so at the 2016 U.S. Open before closing with a 76 to miss out on the title.
Backed by a noisy crowd, Lowry found the rough with his opening tee shot and then his approach rolled back into a green-front bunker, from which he failed to get up and down.
Fleetwood, who is also seeking a maiden major, could have halved the overnight deficit but missed a 10-foot birdie putt and settled for par to stay 12 under.
The next winner of the Claret Jug appeared highly likely to come from that final duo, but J.B. Holmes, Brooks Koepka and Justin Rose would have started the day with other ideas.
Victory for Holmes or Koepka would complete the first American clean sweep of the majors since 1982, handing the former his first and the latter his fifth.
However, Holmes went out of bounds off the first tee en route to a double-bogey six and Koepka also dropped a shot.
Rose, the 2013 U.S. Open winner, remained on nine under through his first two holes.