Martin Kaymer recorded a round of 65 for the second successive day to lead the US Open by six shots at the halfway stage in North Carolina.
Course officials responded to his display on day one by placing the pin in even tougher positions, but the German excelled again on his way to a bogey-free round.
Kaymer's day included two putts from outside 20ft for birdies, while the former world number one also showed his resilience by saving par on a number of occasions.
Brendon Todd moved into second place with a round of 67, but he faces a battle to catch his rival as he looks to win for the second time on tour this year.
Kevin Na and Brandt Snedeker sit one shot further back on three-under, while Henrik Stenson is among a group of players currently eight strokes behind the leader.
