America's big-hitting Cameron Young leads into the second round thanks to firing eight birdies in a flawless 64, a score only McIlroy himself has beaten in the opening round of an Open on the Old Course.

McIlroy's 63 came back in 2010 and was followed by an 80 in terrible weather, while an ankle injury suffered playing football prevented him from bidding to retain the Claret Jug at the same venue in 2015.

The four-time major winner admitted this week therefore felt like the title defence he never had – and he enjoyed an ideal start, holing from 55ft for birdie on the 1st green and making seven in total in his 66, the same score he shot to begin his wire-to-wire victory at Hoylake.

On a day when the early starters took full advantage of benign conditions, including a 67 from Queensland’s Cameron Smith, all eyes were on McIlroy, whose recent major championship form has been impressive. He started with a 65 in the US PGA Championship in May, a 67 in the U.S Open last month and now 66 at St. Andrews.

"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.

“I need to go out and back up what I just did today. I think that's important to do. But again, this golf course isn't going to change that much, I don't think, between today and tomorrow in terms of conditions.

“I've seen the golf course now in tournament play and tournament conditions and know what to expect. Tomorrow's an important run, just to go out and back up what I've done today.”

Rory McIlroy is very happy with his start to the 150th Open at St. Andrews. PHOTO: Getty Images.

The parched conditions meant McIlroy was able to reduce the 614-yard 14th hole to a drive – which went around 400 yards – and a wedge, and he added: "It's the fiddliest Open that I've played.

"Carnoustie was firm in 2018 but it wasn't like this. I think as the tournament progresses, you're going to get some funny bounces and it's going to test your patience at times.

"Fiddly hasn't really been my forte over the years – but I'm hopefully going to make it my forte this week."

McIlroy says he’s been playing well and doesn’t want to think too much about anything other than playing.

“I just have to go out and play the same golf that I've played today, the same golf that I've played over the last few weeks,” he said. “I've been playing well. I've been swinging the club well.

“And I think it's better if I don't think about it that much and I just go out and play golf and try to shoot some good scores on one of my favourite golf courses in the world.”

– Phil Casey