Young of the United States, who is making his debut at the course, impressed after he shot a bogey-free eight-under 64.

His fine run of shots was headlined by seven birdies in his opening 12 holes.

He finished the day with another birdie to close off a splendid round.

McIlroy got off to a flyer when he holed a 55-footer on his opening hole before making three successive birdies from the fourth hole.

He recorded another birdie at the 12th hole, though he registered his only bogey next.

The Northern Irishman, who is searching for his fifth major title, finished strongly to make a six-under 66 and stay within touching distance of Young.

It was his lowest first-round score at the competition since his triumph in 2014.

"Yeah, fantastic start," McIlroy said.

"Just what you hope will happen when you're starting off your week.

"I did everything that you're supposed to do around St Andrews.

"It's another good start at a major, three in a row for me now, and I'm looking forward to the next few days."

Cameron Smith of Australia, and Robert Dinwiddie of England both shot a five-under par 67 to stay hot on the heels of Young and McIlroy.

Those among the group at four-under par are English duo Barclay Brown and Lee Westwood and American Dustin Johnson.

A lot of attention on the opening day was on Tiger Woods, but the American is 14 strokes off the pace in 16th after suffering a torrid day where he made several bogeys, ending at six-over-par.

The winner of this year's championship is set to receive a record prize of $2.5 million (£2.1 million/€2.5 million).

This is part of a substantial 22 per cent rise on last year's prize fund, which now totals $14 million (£11.8 million/€14 million).