Among those trying to chase him down is Paul Casey of England, who was one shot behind.
Casey sandwiched a pair of bogeys around his triple during a four-hole stretch of the back nine.
"I was in the hay too much over the last couple of days", Day said. "When I first played in the GMO's, that was the hardest thing". He hit 12 of the 14 fairways in the second round, and one of the misses came on No. 15, when he was in desperation mode and tried to drive the green on the 357-yard hole. I think the more rounds I can play, I'm hopefully going to get rid of all that stuff and hopefully strip it down to what you saw the last six holes. "Even just trying to take my medicine was very, very hard". It's a good eight in the end.
Elsewhere, playing partners Fleetwood and Harman again matched each other by following up Thursday's 67 with a two-under 70 to reach the half-way stage atop the leaderboard.
Paul Casey is the latest example of how little it takes to record a big number in the U.S. Open.
Hideki Matsuyama vaulted into contention with a 7-under 65, moving to 5 under and a tie for eighth.
"We're in a good spot", Fowler said, referring to his status in the first-person plural after shooting 1-over 73.
"It was still gettable", said Charley Hoffman, who has posted 70-70 and is three shots out of the lead.
That was a 10-shot improvement on his second round and left the 26-year-old on eight under par as he seeks to turn his Ryder Cup heroics into individual success.
"I knew I could shoot a few under par and make the cut", he said.
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"It would be nice to see the course dry out a little bit and just make it to where it's going to take a quality round of golf", he said.
The top two ranked players in the world, defending champion Dustin Johnson and 2011 US Open victor Rory McIlroy, also failed to make the cut. He still went out in even-par 36 and was one shot behind Rickie Fowler, who plays in the afternoon.
McIlroy (78) and Day (79) are coming off their worst scores ever in a U.S. Open, mainly because they couldn't keep it out of the thick fescue that frames the wide fairways at Erin Hills.
Rory McIlroy is further back on 5-over and will miss the cut after carding a 71 in his second round.
It was McIlroy's first tournament since taking time off to tend to a recurring rib injury.
"It's going to be a really cool day for someone tomorrow", Fowler said.
"It's been the best preparation going into a major", Day said.
Fowler dropped to a share of fifth at six under, alongside compatriots Jamie Lovemark and JB Holmes (both 69).
However a bogey on the ninth checked his progress, pushing him out to two over. Despite the bumpy finish, he's a confident man heading into the weekend. "I felt like I've been playing pretty well all week, and didn't have quite the numbers to show for it".
Asked if he would have been able to recover from such a mistake earlier in his career, Casey added: "In my good seasons, yes, but there have been times when I struggled, so probably not!"
With the top 60 and ties qualifying for the final rounds, the cut appeared set to fall at one or two over par.