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.
