McIlroy briefly dropped three shots off the pace when he bogeyed the par-four 12th, his third hole of the day, but bounced back to card 67 at Valhalla.
It was the 25-year-old’s 12th successive sub-par score and a nine-under-par halfway total of 133 means he is a combined 41-under-par for his last 10 rounds, including being 17-under in winning the Open at Royal Liverpool and 15-under to win a first World Golf Championship event in the Bridgestone Invitational on Sunday.
The world No 1 also won the BMW PGA Championship at Wentworth in May, just days after splitting from tennis star fiancee Caroline Wozniacki, even though the wedding invitations had already been sent out.
McIlroy has spoken before of making golf his top priority since then, but also pointed to his collapse to a closing 80 in the 2011 Masters as the reason behind his subsequent success.
“I think I’ve had to learn to be a good front-runner,” said McIlroy, who won his first two majors by eight shots and led from start to finish at Royal Liverpool last month.
“I maybe wasn’t quite comfortable in that position at the start of my career, especially 2011 in the Masters. I was four ahead and I wasn’t quite comfortable in that position.
“It’s taken me a couple of years to grow into that and my mindset has stayed the same since that day at Augusta. If I’m two ahead going into the weekend here, I’m going to try to get three ahead, and if I’m three ahead, I’m going to try to get four ahead.
“I’m just going to try to keep the pedal down and get as many ahead as possible. I went into protection mode once in my career and that did not work out very well so I said would never do it again.”
Speaking about the change in his personal circumstances, McIlroy added: “I think it has happened to me for the better.
“I’ve put a little bit more time into my golf and refocused in a way. It’s the only thing I have, I’ve got my family and my friends, but I just immersed myself in my game. I’ve practiced hard and I’ve done all the right things and I’m reaping the rewards.
“Golf is the No 1 priority to me and while I’m on this run of form I want to try and keep it going as long as possible. I’m going to keep working hard and try and get even better.
“Hopefully, I can do that over the next few years and hopefully you’ll see golf like this more often from me.”
McIlroy had not looked in top form in the early stages of his round, pushing his opening drive into the crowd on the par-five 10th and failing to convert a birdie putt from 10 feet, although a par was at least two shots better than he managed there on Thursday after hooking his second shot out of bounds.
Another birdie chance went begging on the 11th and the world number one then bogeyed the 12th, pulling his drive into the gallery and failing to get up and down from a greenside bunker.
McIlroy, who is looking to become the first player since Padraig Harrington in 2008 to win back-to-back majors, got back on track with birdies at the 13th and 15th and missed from six feet for another on the 16th after a towering four-iron approach from 224 yards.
But he made amends by holing from 30 feet for an eagle on the 18th and could have had another on the seventh, missing from eight feet after a stunning three-wood approach. As if to emphasise a changing of the guard, his closing birdie on the ninth came just as former world No 1 Tiger Woods began his second round on three over par.
“Do I expect to win? No. But do I expect to do the things that I know I can do and control? Yes. And I know that if I do those well, there’s a good chance that I’ll win,” added McIlroy.
Sheffield’s Danny Willett enjoyed another good day after opening up with a 68.
The 26-year-old Yorkshireman hit a two-over 73 to leave him one-under overall.
Elsewhere, US Ryder Cup captain Tom Watson admitted being concerned at the number of his players currently “dropping like flies” and admits new vice-captain Steve Stricker could be required to play at Gleneagles.
World No 6 Matt Kuchar pulled out before the play started on Thursday due to back spasms, while defending champion Jason Dufner lasted just 10 holes before being forced to withdraw with a neck injury.
“We’re falling like flies right now, some of the players that are either on the borderline or on the team,” said Watson.
“That’s a concern because I want those guys to play well?”