The 29-year-old said he played 'no moan' golf as he compiled a second straight 69 in persistent rain Friday morning to sit two shots off the eventual halfway lead of former champion Zach Johnson and fellow American Kevin Kisner at six under.
McIlroy finished one behind England's Tommy Fleetwood and Americans Pat Perez and Xander Schauffele.
Also on four under were last year's runner up Matt Kuchar, fellow American Tony Finau and South Africans Erik Van Rooyen and Xander Lombard.
Tiger Woods carded a second consecutive 71 to end at level par as he chases a first major in 10 years.
Despite sitting six shots back, the former world No.1 insisted: 'I'm certainly right there in it.'
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In 2014 McIlroy was the winner of the Claret Jug at a sun-soaked Hoylake, but he refused to let the conditions get to him on Friday in his chase for a fifth major title and first for four years.
While some players thrive and use bad weather to their advantage, the former world No. 1, now ranked eighth, has not coped well when the elements have turned nasty.
At Royal St. George's in 2011 a disgruntled McIlroy, the newly crowned US Open champion, bemoaned the fierce winds battering the English seaside course and said golf in those conditions wasn't to his liking.
But at a damp and chilly Carnoustie, McIlroy said that while he may not enjoy the adverse conditions any better, he has 'definitely learned to handle it a little better.'
'I just kept level-headed when I needed to,' he told reporters, still wet from his endeavors. 'I didn't let the conditions get to me. I wasn't saying to myself, 'Geez, I wish I was on the other side of the draw.'
'I just got on with it.'
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'Go down swinging'
McIlroy made his splash on the international stage as an 18-year-old amateur when the Open was last staged at Carnoustie in 2007, and he spoke in the build-up about how he hoped to rekindle some of that boyish enthusiasm.
He also stressed Friday the need to unshackle his game and play with more freedom. He thinks that's what may have cost him a tilt at the Masters title in April when he faded in the final group alongside eventual champion Patrick Reed.
'I've been a little too careful and tentative in these big tournaments,' he added.
'I'm committed to making sure that even if I don't play my best golf and don't shoot the scores I want, I'm going to go down swinging and giving it my best.'
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The 42-year-old Johnson won the Open in a playoff at St. Andrews in 2015 after a torrid week of high winds and torrential rain that caused most of the Saturday to be called off, with play running into the Monday for the first time in 27 years.
And the American, who also won the Masters in 2007, showed his fortitude once again by emerging out of Friday's soaking with a 67 to set the pace.
'I'm not going to sit here and say, 'man, I just love playing in the wind and rain,' because I don't, but I can do it. I may not play well, but I'm going to go out and fight,' said Johnson, who is sharing a house with Kisner as well as defending champion Jordan Spieth and a handful of other young Americans in Carnoustie.
He added: 'The reverence I have for this championship, and specifically the Claret Jug, I'm not suggesting someone doesn't have higher reverence for it, but I'd argue with them.'
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Fleetwood is still chasing his first major title, but the fast-rising Englishman, who had to borrow an umbrella Friday as he doesn't have an equipment sponsor and didn't bring his own, set the soggy course alight with a 65.
The world No.10 was second at last month's US Open at a slick Shinnecock Hills, finishing with a major record-equaling 63, but he would love nothing more than a Claret Jug to open his major account.
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'I can't lie, if I could pick one tournament in my life to win, it would be the Open,' he said. 'I've never been anywhere near before, but for two rounds I'm up there on the leaderboard.'
And the American, who also won the Masters in 2007, showed his fortitude once again by emerging out of Friday's soaking with a 67 to set the pace. The 29-year-old said he played 'no moan' golf as he compiled a second straight 69 in persistent rain Friday morning to sit two shots off the eventual halfway lead of former champion Zach Johnson and fellow American Kevin Kisner at six under.