Thomas, decked out in shocking fluorescent pink pants, produced a no less electric performance signing for nine-under 63 to equal the lowest numerical score in a major and the best ever in the 117 year-history of the US Open in relation to par.
But the 24-year-old American will start Sunday's final round trailing by a stroke after Harman turned in a rock-solid five-under 67 to top the leaderboard alone at 12-under 204.
A dizzying day of action could be topped on Sunday with six players within three shots of the lead all eyeing their first major.
Joining Thomas one back are American Brooks Koepka (68) and Englishman Tommy Fleetwood (68) with first round leader Rickie Fowler (68) two adrift and South Korean young gun Kim Si-woo (68) three behind the pacesetter.
The leaderboard may not be packed with household names but the frontrunners bring legitimate credentials to what should be an intriguing Sunday showdown.
Harman, bidding to become the first left-hander to win a US Open, is a winner on the PGA Tour this year while Thomas is the world number 13, a three-time winner this season and a member of the exclusive ’59’ club after breaking 60 at the Sony Open this year.
Fleetwood picked up a victory in Abu Dhabi on the European Tour while Fowler is widely regarded right now as the best golfer never to have won a major.
Players again took full advantage of a softened Erin Hills layout on Saturday, the course left defenceless by overnight rain and only a slight breeze.
They fearlessly attacked the pins and nobody was more ruthless than Thomas, who ripped apart the links-style layout carding nine birdies and two bogeys before capping his record-smashing effort by coolly rolling in an eight-foot putt for eagle at the par-five 18th.
Johnny Miller owned the previous best round of eight-under 63 set en route to victory in the final round at the 1973 US Open at Oakmont.
"I'm a part of history. It means I have a lot better chance to win the tournament than I did when the day started," said Thomas. "I felt like I've been playing pretty well all week and didn't have quite the numbers to show for it. Obviously, today I definitely had something to show for it.
"I had no idea that nine-under was the best ever in an Open, so that was pretty cool once I saw my card.
"The guys at the scoring table told me that, so I was pretty pumped about that.”
