The South Korean rising star finished third in last year's Arnold Palmer Invitational, his first start at the event.
It just didn't feel that way early. He took a penalty drop, hit a pedestrian chip and escaped with a bogey by making a 10-foot putt. "One-over through two instead of two-over is sort of a different feeling".
A bogey on his second hole was quickly offset by two birdies in the final three holes on the front nine, before McIlroy sank three birdies and an eagle on the back nine.
Bay Hill was plenty tough with thick grass framing the fairway and greens that began to quicken under a warm Florida sun. There's so much trouble out there, especially with the wind.
McIlroy could step into Tiger Woods territory today.
"Even par, nothing to get excited about, nothing to rave about", he said.
"One over through six, there's still 66 holes left in this golf tournament", he said.
"You're hitting irons off a lot of tees here".
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And then he was off.
The globe No 309 complied with a 35- footer at the very first by draining pipes a 50- footer at the following and afterwards including a close-range birdie at the 4th to relocate together with McIlroy on top of the leaderboard.
The 36-year-old edged clear at the top of the leaderboard with a three at his penultimate hole after McIlroy - victor in 2018 - had overcome a sluggish start to set the clubhouse target.
His caddie, Harry Diamond, told him it was his best shot of the year.
"I had 260", McIlroy said.
"Sometimes I'm really good, sometimes I'm really bad", he said.
McIlroy was among 10 players who shots in the 60s in the morning.
McIlroy, the 2018 champion, made an inauspicious start when he plunked his approach shot into a lake at his second hole at Bay Hill, but it was as if he was merely giving the field a headstart as he roared back with five birdies and an eagle.
He made careless mistakes that offset four birdies and finished with two wild drives that led to bogeys. "It's not far off but it's still annoying".