The Northern Irishman added a second round of 66 to his opening 64 at the Renaissance Club to be 10 under par, a shot ahead of his Ryder Cup team-mate Tyrrell Hatton (31) and South Korea's Tom Kim (21).
It was another encouraging display by McIlroy, who will return next week to the scene of his only British Open triumph at Hoylake in 2014 - the last of his four major wins.
Friday's round meant that for the fifth time in his PGA Tour career - the event is co-sanctioned with the DP World (European) Tour - McIlroy has started a tournament with two scores of 66 or better.
Four of those previous occasions led to three tournament wins, including his British Open success of nine years ago.
"I hit the ball really well tee to green, gave myself a ton of looks (at birdie)," McIlroy said. "Honestly it felt like four under was probably the worst I could have shot out there.
"But you know, it's swings and roundabouts. I holed a couple of long ones on Thursday to shoot a good score and then missed a couple today, but overall really pleased with how the last two days have went."
With thunderstorms forecast for Saturday afternoon, tee times have been brought forward by several hours, while winds of up to 40 mph are predicted to hit the course on Sunday.
"I don't really have any expectations," McIlroy said. "It would be silly of me to try to pick a potential target score to get to.
"So really just embrace the conditions and just sort of make the best of them."
As for the British Open, he added: "I keep saying that yes, we are all looking forward to next week, but I want to go and win this tournament over the weekend."
Earlier, Hatton gave himself a chance of a first victory in two-and-a-half years with a second round of 62.
Hatton, asked if the round had been as easy as it looked, replied: "No, it was stressful at times, as most of my rounds are.
"But very happy with the chances I gave myself."
World number one Scottie Scheffler (27) and three-time major winner Padraig Harrington (51) are three shots off the lead after rounds of 65 and 66 respectively, with Harrington overcoming wet and windy conditions in the first group out at 07:15 CET.
"I hit a great drive off my first hole of the day, 80 yards short of where I hit it yesterday," Harrington said.
"The marshals were a full 50 yards away from where they should have been for the first three, four holes because they had no idea how short the ball was going," the Irishman added.
