World No.13 Justin Thomas feels ready to win a maiden major championship after shooting a record-setting 63 during a dramatic third round of the US Open at Erin Hills.

The 24-year-old American fired the lowest score relative to par in the 117-year history of the championship, his nine-under round eclipsing Johnny Miller's eight-under 64 at the 1973 US Open at Oakmont.

He'll start Monday just one shot behind unheralded American Brian Harman, whose 67 put him in the lead at 12-under.

Joining Thomas in joint second place is countryman Brooks Koepka and England's Tommy Fleetwood (both 68).

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World No.9 Rickie Fowler (68) is solo fifth, with Players Championship winner Si Woo Kim, from South Korea, a shot back in sixth after also carding a 68.

Whatever happens, the championship is set to crown a first-time major winner, as the closest previous major champion is Louis Oosthuizen in a tie for 17th at four-under. Harman liked his own chances.

"I've got a good plan for this course. I'm confident I'm prepared for whatever happens, good or bad. I'll just rely on confidence," said Harman, who won his second PGA Tour event at the Wells Fargo Championship in May.

With new host host Erin Hills, with its wide fairways and relatively slow greens after rain yielding uncharacteristically low scores for a US Open, Thomas became the 29th golfer to shoot 63 in a major championship.

It is the 31st time a 63 has been recorded at the four big dances, with Greg Norman and Vijay Singh the only golfers to do it twice.

Thomas has had a breakout season on the PGA Tour, winning three times before February, including consecutive titles in Hawaii in January.

His purple patch has instilled the belief he's ready to become the seventh consecutive first-time major winner, a run started by Jason Day at the 2015 US PGA Championship.

"I've felt like my game has been good enough to compete in the majors this year. To have a chance tomorrow is just going to be great," said Thomas.

Kentucky native Thomas became the youngest golfer to shoot 59 on the PGA Tour, which came in the opening round of his Sony Open victory in Honolulu.

"Obviously, I enjoy going low. When I get it going, I don't fear it like I used to of stalling."

Former US Open winners Jordan Spieth and Martin Kaymer were big names who collapsed. Spieth, who won the 2015 event, dropped away with a 76 leaving him in 59th at four-over while 2014 winner Kaymer fell to even-par after a 75.