Billy Hurley III shot a bogey-free seven-under-par 63 to take the midway lead at the Greenbrier Classic but Darren Clarke missed the cut after a second-round 73.
31-year-old Hurley started on the back nine and carded three birdies in four holes from the 14th, and after the turn he recorded six threes and three fours.
His final birdie came courtesy of a superb approach to within six feet of the front-left pin position at the seventh, leaving him in pole position for a first career win at nine under for the tournament.
That would also bring with it qualification for the Open Championship, with four places available to the highest-placed players not already qualified - as long as they finish in the top 12 at White Sulphur Springs.
Clarke, meanwhile, added a second round of 73 to his 71 on Thursday to end up four over, four strokes above the cut.
The top four positions after two rounds were all held by Americans still seeking a place at Royal Liverpool later this month, with Kevin Chappell and Chris Stroud both at eight under and Troy Matteson a further shot back.
Matteson was the biggest mover among the group with a round of nine under par, featuring four birdies in the first five holes and five more on the back nine.
Steve Stricker and Chris Kirk, both already assured of their Open places, shared fifth place with Brice Garnett at six under.
The first non-American challengers, Colombia's Camilo Villegas and Swede David Lingmerth, featured in a group at five under that also included Masters champion Bubba Watson.
Two more major winners, Angel Cabrera and Keegan Bradley, followed in the group at four under.
With America celebrating Independence Day, it was appropriate that Hurley - a US Naval Academy graduate and former lieutenant - was the man setting the pace.
"Anytime you play good, it's great, right?" he said on the PGA Tour website.
"But certainly, there's special days of the year for our country, and being the only person playing this week who has served (in the military), it kind of adds stuff.
"So it's pretty cool, I guess, to shoot seven under on the Fourth of July."
A sizeable field of 69 will continue for the weekend's play after the cut fell at level par.
John Daly, Davis Love III, Webb Simpson and Shawn Stefani were among those to make it right on the line, with Bill Haas, Trevor Immelman, Scott Stallings and United States Ryder Cup captain Tom Watson one shot better off.
Nick Watney, Marc Leishman and Ricky Barnes were all one stroke outside while John Peterson reacted with undisguised anguish after his par putt at the last lipped out and cost him his place for the weekend.