The top 18 players on the leaderboard going into the weekend had combined for zero majors.
None of the leaders had as shaky a stretch as Casey, who made 8 on the par-5 14th hole - after hitting into the fescue and almost whiffing on his swing to get out - and bogeyed the next hole.
"If you look at the golf course and you even talk to me, Jason or Rory, this course sets up ideal for us", Johnson said.
"If you look at the golf course and you even talk to me, Jason or Rory, this course sets up flawless for us".
The future could be now for Casey who starts the day at seven-under in a four-way tie for the lead alongside compatriot Tommy Fleetwood and Americans Brooks Koepka and Brian Harman.
He was at 7-under 137, two shots clear of anyone else who played early in warm sunshine on a course that getting dry and crispy.
Fellow Australians Wade Ormsby (six-over after a second 75) and Nick Flanagan (10-over after a 79) also went home early, joining Johnson (four-over after a 73) and four-time major victor McIlroy (five-over after a 71). Five players are at 5 under - Si Woo Kim (70), Xander Schauffele (73), amateur Cameron Champ (69), Brandt Snedeker (69) and Hideki Matsuyama, who tied for the low round of the day with a 7-under-par 65.
"Tomorrow will be a very cool experience", Fleetwood said.
After tying for 45th at the Memorial last month, Matsuyama arrived at Erin Hills on a downturn in a topsy-turvy season.
"Even though it's very disappointing to not be here on the weekend, I think these last two rounds will serve me well going into the summer", added the 28-year-old, who has added the Scottish Open to his schedule in between the Irish Open and Open Championship.
Casey took his place amongst the leading group after recovering from an early implosion that sent him plunging down the leaderboard.
However the 39-year-old world number 14 unfurled a sequence of five straight birdies to haul himself back up the leaderboard, carding a one-under-par 71 to finish the day at seven under.
"I was in the hay too much over the last couple of days", Day said.
Garcia carded an eagle, three birdies and three bogeys in a roller coaster round.
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"I hate just being in the situation where I've been in a good spot and maybe let it get away from me a little bit and let some mistakes compound and turn a round that you (just) kind of fight through it and end up being in a decent spot after the day", Fowler said.
"I couldn't possibly shoot any higher than I did", he said.