The Northern Irishman, starting on the back nine, got his round going with a chip-in birdie at the par-five 13th having blasted his greenside bunker shot into the far fringe.
Two more birdies saw him turn in 32 but he then rattled off four more in seven holes - including a 48-footer on the seventh to take a share of the lead with Xander Schauffele.
However, he saved his best until the par-four last when he cut the corner with a 332-yard drive to leave him 44 yards to the pin and he chipped up for a tap-in birdie to move to eight under, one shot clear of Schauffele and Scotland's Martin Laird.
McIlroy may have only hit seven of 14 fairways but he was happy to have maintained form built up after winning the Canadian Open two weeks ago and then finishing joint fifth in the US Open.
"I think the good thing is I've been playing well and you want to ride that wave of momentum you have," he said in his post-round interview.
"Coming in here fourth week in a row I knew I was going to be a little bit tired but whenever you are playing this well you just want to keep it going.
"If I had taken this week off I'd have been sitting at home thinking it was a missed opportunity.
"My preparation hasn't been what it would usually be. I got off to a nice start, hit some quality shots early on which gave me some confidence and I built upon that.
"When I got into the second nine I started to play really well and started to hole some putts and when you get on a run like that you just have to try to keep it going."
Schauffele was another player for whom preparation was not what he wanted as a mix-up over tee times meant his arrival at TPC River Highlands was slightly more rushed than usual.
However, it did not affect the American as he hit 18 greens in regulation in a bogey-free 63, a round matched by Laird.
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