Masters champion McIlroy is playing only his sixth regular PGA Tour event of the year at the tournament in Muirfield, which is hosted by 18-time major winner Jack Nicklaus.
The Northern Irishman, who tied for seventh place at last month's PGA Championship, continues to tailor his schedule to his personal circumstances – with focus soon turning towards the upcoming US Open at Shinnecock Hills, New York.
McIlroy had gone into the water at the third for a double bogey, but then responded with a run of three birdies to reach the turn at one under on Thursday (Friday AEST).
A hooked tee-shot off the 13th put the 37-year-old out in the deep rough on the left, but a fine recovery saw him save par, before a superb approach inside two feet at the 14th then set up another birdie.
The 18th saw the world No.2 hit another wayward tee-shot off into the left rough, narrowly missing the water, before looking to get up and down. However, he just failed to roll in a seven-foot putt to save his par and signed for a 71.
"I felt like I played pretty well, for the most part, especially after making that double (bogey) on the third hole, to play the rest of the round in three under was good," McIlroy said.
"It is a tough course, you have to concentrate on every shot. The greens are very firm already, very fast, (you) get yourself above the hole, it is very tricky."
Tommy Fleetwood was in a group of four who share the lead at five under alongside Americans Wyndham Clark, JJ Spaun – the 2025 US Open champion – and Ryan Gerard.
Canada's Nick Taylor sits at four under, recovering from a double bogey at the par-five seventh.
Justin Rose is a further shot back alongside Sam Burns, with Ireland's Shane Lowry in another group at two under.
Adam Scott shot an even-par 72, while fellow Australians Jason Day and Min Woo Lee carded 74s.
Scottie Scheffler is looking for a third successive win at the tournament, but laboured to a one-over round.
US PGA Championship winner Aaron Rai was another who failed to make an impact on the leaderboard after also making 73 playing alongside the world No.1.
With AAP.
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