McIlroy lead cut to two shots at Open
Northern Ireland's Rory McIlroy plays from the rough during his second round 66, on day two of the 2014 British Open at Royal Liverpool in Hoylake, on July 18, 2014 - by Peter Muhly
The Northern Irishman, who has a reputation of not liking wet conditions, bogeyed the first hole to fall back to 11-under while playing partner Johnson rattled in a birdie to immediately pile the pressure on the 25-year-old leader.
Another American, Rickie Fowler, moved into third on eight-under as he got off to a flying start with two opening birdies.
The course which was drenched by overnight rain that turned to late morning drizzle was giving the players plenty of birdie opportunities as soft greens made ideal conditions for low scoring.
Jim Furyk, who came into the day, seven shots back of McIlroy also came flying out of the blocks with birdies at three and four to move to seven under and sole possession of fourth place.
A trio of players including American Jimmy Walker, Charl Schwartzel of South Africa and Spaniard Sergio Garcia were a further shot back and within five shots of the Ulsterman.
The field began at both the first and 10th holes for the first time in Open history as atrocious weather overnight and an expected afternoon storm forced organisers to reorganise the schedule.