World number one McIlroy, who has won The Open Championship and WGC-Bridgestone Invitational in consecutive starts, began his bid for three victories in a row in Kentucky with a first-round, five-under-par 66.

That leaves the Irishman just one stroke behind 18-hole leaders Lee Westwood of England and American duo Ryan Palmer and Kevin Chappell, all of whom carded six-under 65s on day one of the final Major of the year.

That trio will take a one-shot lead into Friday's second round, with Jim Furyk of the United States, Italy's Edoardo Molinari, Englishman Chris Wood, Henrik Stenson of Sweden and McIlroy breathing down their necks on five-under having carded opening 66s.

McIlroy's round included eight birdies but also featured a double-bogey seven at the 10th followed by a three-putt bogey at the par-three 11th.

Earlier this season, that wobble might have been the cue for McIlroy to unravel in the way he has played himself out of contention with stretches of poor play and high scoring in a round.

This time, his confidence renewed by a big improvement in his putting and some prodigious and accurate driving off the tee that has helped deliver two of the season's biggest prizes to go with his BMW PGA Championship won in May, the 25-year-old deployed his new-found mental strength to rattle off four birdies in a row before completing a flawless final seven holes with another birdie at the last to sign for his 66.

Having looked nicely poised at three under par following a bogey-free front nine, the three-time major winner saw his supreme confidence tested after sending down a 315-yard drive to the middle of the par-five 10th fairway when his second shot at the par-five 10th went out of bounds, way left of the green and he ended up with a double-bogey seven.

That was quickly followed by a bogey at the par-three 11th, his tee shot to the centre of the green followed by a three-putt from 46 feet.

“What happened on 10 really knocked me off track a little bit and it’s one of only a few bad shots I’ve hit in weeks,” McIlroy said.

“I just double crossed my second shot and what I was really angry about was that you make a seven but you don’t compound that error by making a bogey on the next hole which is what I did.”

That sent McIlroy back to level par for his round but this time, instead of crumbling as he might have down earlier this season, the response was altogether more satisfying, the Irishman nailing his 176-yard approach at the par-four 12th to four feet from the pin and responding with a heartfelt fist pump as he got back on track with a birdie, followed by two more, his hat-trick sending him back to three-under.

“Walking onto the 12th tee I was muttering a few things to myself but it was just great to hit that second shot in close, in the way I did, as it is one of the toughest par fours on the course and to make birdie, that made me feel a little bit better,” he said.

“And after what happened at 10 and 11 you have to take whatever you are feeling inside and turn it into a positive because I was hot and it was trying to use that fire as a fuel to propel yourself forward.

“So it was great and it shows where my game is at the moment as I was able to do that.”

McIlroy was in the groove. At the 14th he had sunk a 29-foot putt for birdie and once the final ppar-three was out of the way it was the turn of the big stick to do the damage, McIlroy unleashing drives of 304, 340, 335 and 322 yards off the remaining tees, all of them finding the fairway.

They led to a birdie at the 15th and an eagle chance at the par-five last, McIlroy's putt from 31 feet just failing to rope. There was an agonised look about the Holywood star as the putt just broke away from the hole at the death but the birdie was enough to push him a shot closer to the leaders.

“One of the big things for me in major championships is to get off to a good start and if you look at my three wins in the majors I have always got off to a good start and it was important to do that today and hopefully I can continue to be up there for the rest of the tournament,” McIlroy said.

“It could have been a very special round today but I will take what I have been given and enjoy my dinner tonight.”

McIlroy was not the only Irishman to have earned the right to a good night's sleep with the in-form Shane Lowry picking up from where he left off in making his first start since a tie for ninth at The Open, his best finish in a Major.

Starting from the 10th, Lowry bounced back from a bogey at the 217-yard par-three 14th by rattling off three birdies in a row around the turn.

A bogey five followed at the second but Lowry bounced back with birdies at the fourth and seventh holes to get to three under and card a very satisfactory opening 68.

There was only frustration for the rest of the Irish contingent as Padraig Harrington and Graeme McDowell carded opening 73s, two over par while Darren Clarke signed for a 79.

Four-time champion Tiger Woods will start his second round on three over after a 74 in the company of Harrington and Phil Mickelson (69) but Jason Dufner is no longer able to defend the title he won 12 months ago at Oak Hill, the American managing only 10 holes before withdrawing due to a long-standing neck injury.