"You just have to make the adjustments". He chipped in for par on No. 10, and made eagle on No. 15 when he lashed a three-wood to the center of the green on a 542-yard par-five hole.
Paul Casey has had better birthdays, and on the occasion of his 40th the Englishman, who was tied for fourth at four under overnight, slumped to a 77 and finished the day at three over after waiting nearly four hours to register his first and only birdie, at the par-five 15th.
The younger pair both won their final tournaments before landing in Lancashire - Koepka's being the last major, the US Open at Erin Hills - and shot five-under 65s in the tougher conditions of the morning play.
The Ryder Cup specialist, who has never won a major, endured a frustrating 2016 due to injury problems and had to come through qualifying to earn his place in the tournament.
"If it's an afternoon round and the other side has already played the morning, that's when it's tough", he said.
But US Open champion Brooks Koepka is adamant it's not where you're from but how you think.
South African Ernie Els and BMW PGA champion Alex Noren are among those on two under, three back, alongside World No 2 Hideki Matsuyama.
He will be playing in the final group of the fourth round at a major for the first time, and the 39-year-old Kuchar sounded up for the occasion.
"It feels absolutely marvelous, it really does". "I don't really care that much".
In the past was his last time leading a major, when he let a five-shot lead get away from him on the back nine a year ago at Augusta National.
After rallying with four back-nine birdies in an opening 71, the Northern Irishman immediately picked up from where he left off to storm into red numbers.
After seven straight first-time major winners, the 146th Open Championships seems due to break the streak.
That saw him drop back to four under par, although in increasingly heavy downpours, the world number three then saved par at the 10th by holing a chip from just off the back of the green.
The 27-year-old was four under par through 13 holes before dropping a shot at the 16th.
He went out in the afternoon and, in pouring rain, promptly birdied the hard par-four 1st hole. However, it was no surprise to see McIlroy get up and down once again, as he showcased a much sharper-looking short game to his army of fans, more of whom were flocking to see his charge through the field.
But, with the likes of Japan's world number two Hideki Matsuyama looking a threat, can another first-time victor lift the Claret Jug in front of Birkdale's distinctive white Art Deco clubhouse on Sunday evening?
It was then that the wind picked up and McIlroy started to lose control of his ball in the hard crosswinds.
"I played well today and I'm certainly not out of it, although I would like to be a little closer", said Kuchar, who is seeking his first major title.
SHOT OF THE DAY: Charley Hoffman hit an iron from the right rough into the hole on No. 1 for an eagle. "A couple of putts slipped by the edge but it adds up to a decent score so I'm pretty happy". I just have to try to keep that going for the next two days. And thought to myself, I am glad I'm over here, glad I'm doing this. "I essentially missed two greens today in some 15mph winds".
"But I enjoy it because as long as I approach it positively and recognize that this is what you want to feel because you're in a position you want to be in, then the easier it is to hit solid shots and to create solid rounds".