Thomas was already on course for a share of the lead as he stepped up to the 18th having carded nine birdies against two bogeys in a spectacular round.
Japan's Hideki Matsuyama was battling his way into contention going four-under through 13 holes to sit four off the pace alongside Englishman Tommy Fleetwood.
Erin Hills made its debut as a U.S. Open course Thursday with a most gracious welcome for Rickie Fowler, who matched the record to par in the opening round with a 7-under 65 on the longest course in major championship history.
Playing in the final group with Harman will be Thomas, who is almost as small but one of the longest hitters in the game.
American Justin Thomas became the 29th golfer to shoot 63 in a major championship. A week later, at the Memorial, Spieth shot a 66 in the first round before fizzling and tying for 13th. About all that might save Erin Hills from another onslaught was a forecast for the strongest wind of the week.
It infuriated Harman, and he played the rest of the match in a cold rage, making three straight birdies to win 1 up on the 18th hole.
I want a U.S. Open where fairways are the width of hotel corridors and greens are the consistency of trampoline webbing.
Spieth is well off the lead of 12 under held by Brian Harman. I think I hit 17 greens today which was just awesome in these conditions. "But it isn't a U.S. Open course that I'm familiar with the way it was set up". Just yesterday I brushed Harman's chances aside as we rolled into the weekend.
"Yeah, first cut made, too (at the U.S. Open)", Harman said.
Wind was supposed to be one of the main defenses at Erin Hills, which is the longest U.S. Open course in history but also has the widest fairways the tournament has seen.
The soft, forgiving conditions created opportunity for major movement up the leaderboard for those players willing to take it. "I don't mean that in a bad way".
Most unusual about Saturday is the four-way tie for the lead among Paul Casey, Brooks Koepka, Tommy Fleetwood and Brian Harman.
"Tomorrow will be fun and special, just going around here and being a part of this first U.S. Open here in Wisconsin", Stricker said.
China's Li Haotong demonstrated what could go wrong as he struggled to an 84 which contained two birdies, two double bogeys, a triple bogey and seven bogeys, six of which came in succession from the third. His lone victory in America was the Houston Open in 2009, though not to be overlooked are his 14 victories around the world that include two victories in Abu Dhabi, one at the BMW PGA Championship, two at the Johnnie Walker Championship at Gleneagles and the World Match Play Championship that included Tiger Woods.