Fowler, who missed the last two US Opens as he fell to as low as 185th in the world last year, launched his return to the tournament with a breathtaking, record-setting eight-under par 62 on Thursday at Los Angeles Country Club.
He followed up with a two-under 68 for a tournament record-equalling 36-hole total of 130, finding eight birdies amid six bogeys as the course finally began to firm up and look like a classic US Open challenge.
"It's not that easy out there," Fowler said. "Yes, I've made a lot of birdies, and that is doable out there.
"It's still a very hard test. You hit fairways and hit greens, yes, you can score well, but you get out of position and it's going to eat you up."
In hot, sunny conditions, the LA Country Club North Course, hosting the US Open for the first time, was expected to provide the test of patience and nerve typical of the tournament.
World number four Patrick Cantlay, who made the cut on the number at two-over, showed birdies were available with five on the more inviting front nine, while 2019 British Open champion Shane Lowry had four birdies in the first nine.
In an expected twist, the 15th was set up to play at 81 yards - making it the shortest par-three in US Open history.
The hole played at 124 yards and 115 in the first and second rounds and yielded three holes in one.
Fowler has little margin for error. He led American Wyndham Clark by one stroke, with four-time major winner Rory McIlroy and 2021 Tokyo Olympic champion Xander Schauffele two shots off the lead.
American Harris English, like Fowler and Clark seeking a first major title, was alone in fifth on 133, one stroke in front of two-time major winner Dustin Johnson and Australian Min Woo Lee.
World number one Scottie Scheffler was five shots adrift, tied with rising US star Sam Bennett and one shot in front of British Open winner Cameron Smith of Australia.
Fowler is aiming to cement his return to the form he showed in 2014 when he tied for second at the US Open and finished in the top five in every major.
Heating up
He would add another half-dozen top-five major finishes to his resume, but never lifted a trophy. As he struggled in 2021 and 2022, it looked as if his window to claim one had slammed shut.
But Fowler has six top-10 PGA Tour finishes this season, including a runner-up at the Zozo Championship last October, a tie for sixth at Colonial and a tie for ninth at the Memorial in recent weeks.
Even so, Fowler was well aware that the halfway lead was no guarantee on a course set to play tougher every day.
"Having a lead right now doesn't really mean much," Fowler said on Friday. "A little different once you get to after 54 holes because that's when things really heat up."
Fowler was due to tee off on Saturday at 3.40pm (11.40pm BST) alongside Clark, who earned his first US PGA Tour title at Quail Hollow last month.
McIlroy, trying to end a nine-year major title drought, teed off alongside Schauffele at 3.29pm (11.29pm BST).
