Johnson shot a second-round 74 in brutal conditions to finish at -1, alongside Patrick Reed, Matt Kuchar and Hunter Mahan.
McIlroy is one shot back alongside fellow Ryder Cup stars Graeme McDowell - whose 71 was one of the best rounds of the day - and Francesco Molinari, with Wales's Jamie Donaldson also level par.
Tiger Woods is at +5 after a solid second-round 73 that kept him in contention - thanks in part to a 91ft putt on the 4th hole.
- Sports & Recreation
- Matt Kuchar
- Francesco Molinari
- Patrick Reed