Paul Casey tumbled dramatically down the leaderboard on the second morning of the U.S. Open, as Kim Si-woo got to within one of leader Rickie Fowler at Erin Hills.
Casey and unheralded American Xander Schauffele came closest to catching Fowler on day one, after the latter had set the pace with a sparkling 65.
When Casey birdied his second hole on Friday, the 11th, he was alongside Fowler at seven under for the tournament.
However, the Englishman then bogeyed the next hole before carding a damaging eight on the par-five 14th. Another dropped shot at 15 left him five off the pace.
Players Championship winner Kim, who also started on the 10th, enjoyed contrasting fortunes, three successive birdies from the 17th lifting him to three under for the day and six under overall.
Schauffele remained one off the lead after four holes of his second round, while the three players on five under overnight - Brian Harman, Tommy Fleetwood and Brooks Koepka - were all among the later starters on a day when higher scoring was anticipated. Fowler was due to begin his round at 1:36pm local time.
Rory McIlroy and Jason Day each appeared destined to miss the cut. The high-profile duo were six and five over respectively for the tournament after seven holes on Friday, although Day had at least improved his position after a first-round 79.