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Young carded seven birdies in 12 holes first and also missed a couple of relatively short putts at the par-five 14th and 15th which would have got him to nine under.
But McIlroy was hot on his heels with a birdie attempt to also turn in 31 just sliding by, although he would have been more than satisfied with his four-under outward half.
The Northern Irishman had the ideal start as his putt from almost 60 feet dropped at the first.
He then reeled off three successive birdies from the fifth, where he picked up a shot despite his drive losing some yardage after hitting a stone marker in the middle of the fairway which designates the original boundary of the course.
With little wind and a dry, fast-running course organisers the R&A did their best to hide pins on the greens in order to provide some protection to the 7,313-yard Old Course.
But there were still plenty of scoring opportunities to be had with a number of par-four greens reachable off the tee.
Even the 53-year-old Paul Lawrie, champion at Carnoustie in 1999 and given the honour of hitting the first tee shot at St Andrews for the second time in 12 years, took advantage with his drive at the downwind 356-yard last finishing five feet from the hole for an eagle as he signed for a two-over 72.
LIV Golf rebel Ian Poulter turned jeers into cheers as, having been booed by some fans on the first tee for his involvement in the Saudi-backed breakaway series, he was playing to the galleries by the par-four ninth hole when he holed a massive 160-foot putt for the tournament’s first eagle.
Ever the showman Poulter milked the applause, standing with his hand up to his forehead peering into the distance in the direction of where his ball had disappeared into the hole.
That briefly catapulted him into a share of the lead on three under, which is where he finished after a round of 69.
Poulter’s round had begun inauspiciously as he almost missed the widest fairway in golf, pulling his mid-iron well left across the 18th fairway before his ball finished just a couple of yards short of the out of bounds line which borders The Links road on the fringe of the course.
There was disappointment for his Ryder Cup team-mate Justin Rose, as he was forced to pull out with a back injury just before his scheduled 8.14am tee time.
The 41-year-old had ended Wednesday’s practice round prematurely, leading to speculation he may have sustained a problem, and when he did not arrive for his scheduled slot, Japan’s Rikuya Hoshino took his place alongside the Ryder Cup dream team pairing of Tommy Fleetwood and Francesco Molinari, the 2018 Open champion.
First reserve had been Alex Noren but he chose to go to the PGA Tour’s Barracuda Championship in California, which starts later on Thursday.
“On the 4th hole yesterday during my practice round, I made a swing with my driver and started to feel pain in my lower back,” Rose wrote on Twitter.
“I’ve been getting around the clock treatment but it just doesn’t feel ready to compete in this prestigious championship.”
Britain and Ireland Walker Cup amateur Barclay Brown was in the clubhouse at four under after a round of 68, which included a birdie at the notoriously difficult 17th Road Hole.