Johnson was often 50 yards behind McIlroy off the tee at Firestone Country Club, but carded a 65 to the Northern Irishman's 68 to finish nine under par alongside Belgium's Thomas Pieters. "When my pace starts to feel good and I start to get dialed in, that is when I know I am hitting it good".
McIlroy's victory at Firestone in 2014 was sandwiched between major titles in the Open Championship and US PGA, but the 28-year-old has failed to win a major since and split from his long-term caddie JP Fitzgerald after finishing fourth at Royal Birkdale last month.
A good performance at Quail Hollow for McIlroy may well lead to us seeing a lot more of Mr Harry Diamond in the coming years.
Another stoppage in play while on the 10th hole proved to be the longest of the day with the players off the course for a number of hours. "I just keep plugging away".
Johnson, who won the Masters in 2007 and the 2015 Open Championship at St Andrews, carded eight birdies and three bogeys in his 65 to briefly take the outright lead, until Pieters also birdied the last.
"I was three behind going into Sunday last time and I think the took the lead by the sixth tee box", said McIlroy.
"I was pretty awful from the tee, but managed to hit a lot of greens".More news: Liverpool predicted line-up: Jurgen Klopp to start strongest team against Athletic Bilbao?
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It was a similar story on the back nine and, by the end of the round, he had hit drives of over 300 yards at every hole bar the par threes.
"I played decent", Pieters said.
Pieters drained a 30-foot birdie putt on the 18th hole to cap a 4-under-par 66.
Phil Mickelson and Australian Jason Day both shot 71 and are tied for 33rd.
The two-time major victor outshone playing-partner McIlroy, particularly with the putter, although his round was not without its blemishes as he mixed eight birdies with three bogeys overall and carded only one par from the fifth through to the 11th.
"My ball striking was good, driver was not good. and I managed to score really well, so pretty happy", said Pieters, a three-time victor on the European tour. "I didn't help myself by missing a few chances as soon as we came back out".
But the reigning PGA champion lifted his spirits with a faultless inward half, and birdies at 15 and 16 hoisted him back to four under for the tournament.