Rory McIlroy looked ahead to Friday and admitted he needed to "go out and back up" his impressive start to the 150th British Open after an opening 66 at St Andrews placed him among the leaders in the clubhouse.

The Northern Irishman's six-under-par first round left him just two shots behind afternoon clubhouse leader Cameron Young of the United States, who enjoyed a 64. McIlroy birdied the first and last holes of his round, and also racked up three straight birdies between the fifth and the seventh.

The favourite with bookmakers to lift the Claret Jug, his only dropped shot for the day came at the par-four 13th hole, but he enjoyed birdies either side of that, at the 12th and at the long 14th.

"I came in here playing well, and I've played this golf course well over the years. So I knew if I just went out there and played my game and stuck to my game plan, that something like this was possible," said the world number two.

"I need to go out tomorrow and back up what I just did today. I think that's important to do.

"I've seen the golf course now in tournament play and tournament conditions and know what to expect."

McIlroy, who said the conditions at the Old Course made this "the fiddliest" Open he has played, has not added to his tally of four major titles since winning the PGA Championship in 2014.

He came second at the Masters in April, falling short of winning the one major he needs to complete a career Grand Slam.

Since then he has won the Canadian Open and tied fifth at the US Open, confirming his fine form heading to St Andrews.

"Everything feels very settled. No real issues with my game. Everything feels like it's in good shape," he said.

"Everything feels just sort of nice and quiet, which is a nice way to be."

Now 33, McIlroy won the Open at Hoylake in 2014 but missed the chance to defend his title the following year in St Andrews after suffering an ankle injury playing football.

He had joked about wrapping himself in cotton wool to ensure he remained fit this time, and McIlroy admitted nerves were inevitable on the first tee on Thursday morning.

"As first tee shots go, it's pretty easy, hitting a 4-iron into a 120-yard fairway," he said.

"But there's always nerves. There's always anticipation, I guess, and wanting to get off to a good start.

"But once that first tee shot's gone and out of the way, you sort of get into your routine and you're just playing golf. And that's where I feel most comfortable."