The 30-year-old has moved on to 12-under, putting him one shot clear of fellow Americans Justin Thomas and Brooks Koepka who are tied for second place alongside England's Tommy Fleetwood.

Harman posted a blemish-free back nine, which included birdies at the 11th, 14th and 15th before he narrowly missed out on another at the 18th.

Thomas became the first man to shoot a nine-under-par round at the US Open and claimed the fifth 63 in its 117-year history.

Other players to have carded rounds of 63 include 18-time major winner Jack Nicklaus, who did so on the par-70 Baltusrol course in New Jersey in 1980.

Compatriot Tom Weiskopf achieved the same feat at the same event, which came seven years after the US's Jonny Miller hit the first 63 on the par-71 Oakmont course in Pennsylvania in 1973.

Fiji's Vijay Singh, meanwhile, managed a seven-under 63 at Olympia Fields in Illinois in 2003.

Thomas' fellow countryman Rickie Fowler, who led after round one, is two shots off the pace on 10-under.

He had a run of three consecutive birdies from the 14th to the 16th on his way to shooting a four-under 68.

South Korea's Kim Si-woo, who became the youngest winner of the Players Championship last month, is on nine-under after posting a 68.

Tied for seventh place on eight-under are the American trio of Charley Hoffman, Patrick Reed and Russell Henley.

England’s Paul Casey started the day on seven-under, but fell away with a three-over 75 and is tied for 17th spot on four-under.

Among the other eight players on the same score as Casey is Spain's Sergio Garcia, the Masters champion who registered a one-under 71.

Action at the major in Wisconsin is due to conclude tomorrow.