"I think the golf course is gettable", said Johnson, who played in the morning.
That has helped attract a 156-player field that includes five world's top 10 ranked golfers and all four reigning major championship winners: Brooks Koepka, who won his second straight U.S. Open last week; Spieth; Masters victor Patrick Reed; and PGA Championship victor Justin Thomas.
David Duval, the 2002 British Open champion, believes Spieth is overdue some time off to recharge. "I don't know what it is about the hole, but that hole, I get on the tee and for whatever reason I struggle".
The Texan (24) had a triple bogey eight after driving out of bounds and finding water at the 13th, his fourth hole, and needed a tap-in eagle at his 16th to salvage a three-over 73.
McIlroy birdied the opening two holes and grabbed three-in-a-row from the 13th onwards, but he missed out on joining Spieth and Johnson in setting the pace after a bogey at the last. "It probably would have gone four or five feet by, but I couldn't hit it any better than I did". He now holds a four-shot lead with one round to play.
"You're anxious about in the back of your head is Michael going to go chest bump or side bump?"
McIlroy shares third with Peter Malnati and Brian Harman, while England's Paul Casey continued his fine form on the PGA Tour this year as he returned a six-birdie 65, with Padraig Harrington one further back after the Irishman kept a bogey off his four-birdie card.
McIlroy hit 17 greens in regulation on Thursday. Koepka, who started on the 10th tee, shot a 31 on his first nine, but bogeyed four of his final six holes.
"Still don't really know where I'm at", a tired Koepka told reporters. "But I'll be honest, I'm dead right now".
"They've got some kind of magnetic sand for me", Spieth joked.
Harman, a runner-up at last year's US Open, seeks his third US PGA crown after the 2014 John Deere Classic and last year's Wells Fargo Championship. That has helped attract a 156-player field that includes five of the world's top 10 ranked golfers and all four reigning major championship winners.
No player is coming in as hot as last week's victor, Brooks Koepka.
It is not the first time Spieth has been undone by the hole.
Thomas said he, like most others, isn't just using this to tune up their game for the next major. "I think out of instinct went to give me the putter and I told him I didn't need that, so put it back in the bag".