The 29-year-old American maintained his advantage from day two and has one hand on another major crown at Bethpage State Park Black Course in New York.

After stunning rounds of 63 and 65 on days one and two, Koepka carded a level-par 70 third time around.

But his rivals were unable to take advantage with his lead remaining colossal as he chases the fourth major of his career.

Koepka, who had achieved the lowest 36-hole score in major championship history after day two, did not register a bogey in his first 28 holes but did so on both the ninth and 10th today.

He did the same at the 16th but his round also included three birdies as he seeks to reclaim the title he won at the Town and Country in Missouri last year.

As the 2017 and 2018 champion of the US Open, Koepka would become the first player to hold back-to-back crowns in two majors should he finish the job tomorrow.

It would also mean a fourth major in his last eight attempts.

"I'm definitely not going to let up, I promise you that," he said.

"I think I'm more focused than anybody out there. 

"I think I'm tunnel-visioned.

"I couldn't care less about breaking records.

"If they come that's awesome but I'm just trying to win.

"I feel confident, I feel good and I feel excited."

Four players sit on five-under-par behind Koepka, including American world number one Dustin Johnson.

A bogey on the final hole denied Johnson, the US Open champion in 2016, the chance to go second on his own and he will not now get the chance to put the pressure on Koepka from the final pairing.

Fellow Americans Harold Varner III and Luke List, and Thailand's Jazz Janewattananond, are also seven behind the leader.

Major winners Adam Scott and Jordan Spieth started day three as Koepka's nearest challengers but both faltered.

Spieth, who needs to win this tournament to complete a full set of majors, carded a 72 and slipped back to three-under.

Australia's Scott, the 2013 Masters champion, delivered an identical round with the pair now part of a four-way tie for eighth.