Poor visibility had again delayed the start of play for nearly three hours, with 60 players waiting to complete their second rounds.
That made for a strange day’s golf as the likes of McIlroy, U.S. Masters champion Jordan Spieth and world number six Fowler played nine holes in the morning before returning to the course late afternoon.
Four-times major winner McIlroy grazed the pin from nearly 270 yards on the final hole of his second round, eventually making an eagle.
That followed a birdie on 17 — his first of the round — to catapult the Northern Irishman level with U.S. amateur champion Bryson DeChambeau on eight under and into the tournament-leading trio alongside Sullivan, 28, who was 10 under.
“I couldn’t hole from outside two feet so I thought I’d hit a couple of shots within two feet on the last two holes. It glossed over a pretty average round,” said the 26-year-old McIlroy.
Returning to the National course, McIlroy sunk a six-foot birdie on the second hole.
DeChambeau, in contrast, was loose from the tee and fairway and careless in his putting, slipping to seven under on the same green after a wayward four-footer.
Sullivan also struggled with his long game, bogeying the third to slip level with McIlroy on nine under. This duo then birdied four, before Sullivan seized a two-stroke lead with successive birdies at seven and eight.
But on nine, the world number 37 was left forlornly searching bushes lining the water after an errant drive.
The ball lost, Sullivan’s misery deepened as his second tee shot found the rough and the next skidded into a bunker as he made a triple bogey to tumble back to nine under.
McIlroy and American Fowler were both 10 under after nine and 11 holes respectively, sharing the lead with South Africa’s Branden Grace (16 holes), England’s Ian Poulter (13 holes) and Dutchman Joost Luiten (10 holes).
“The game feels really good and I’m swinging really well,” said Fowler, who is four under for the round following scores of 70 and 68 in the opening two.
“I rounded into form as the week has gone on. It’s going to be a long day tomorrow.”
(Reporting by Matt Smith. Editing by Patrick Johnston and Ed Osmond)