The former world number one produced the best round of the day, 66, to take a one-shot lead into the final round, having produced five birdies and only one bogey despite the conditions.

"It [winning] would mean a lot," Koepka said.

"I think a major championship would mean a lot to anybody.

"To win one would be fantastic.

"Just got to go out and go play good tomorrow."

Koepka heads into the final round on six under and a shot clear of Corey Conners of Canada, who held a two-shot lead after the seventh but then suffered a costly double-bogey late in his round.

Viktor Hovland of Norway is also a shot back after a final-hole bogey.

Another home player, Bryson DeChambeau, moved up to third place on -3.

Meanwhile world number two Scottie Scheffler, who had gone into the third round as joint leader with Conners and Hovland, dropped down to joint fifth place with England's Justin Rose on two under.

Northern Ireland's Rory McIlroy moved to within five shots of the lead thanks to a second successive 69 as he chases a first major victory since 2014.

McIlroy was briefly three back after birdieing both early par-threes but lost ground with three bogeys in a four-hole stretch, then followed three birdies in five holes on the back nine by dropping a shot on his penultimate hole of the day.