Rory McIlroy
World number two McIlory and US Open winner Rose struggle on first day of The Open. (Reuters)

World number two Rory McIlroy endured a disastrous start to his Open campaign finishing the opening day eight over 79, 13 shots off American Zach Johnson who tops the leader board with an impressive five under 66 at Muirfield.

US Open winner Justin Rose also failed to shine on the opening day as he lumbered to a four over 75 as 2007 Masters winner Johnson withstood the challenges of 56-year-old Mark O'Meara and Rafael Cabrela-Bello who finished four under, ahead of Dustin Johnson, Brandt Snedeker and Tom Lehman, Shiv Kapur and Miguel Angel Jimenez who all ended the day three under.

World number one Tiger Woods had to re-tee on his first hole upon his introduction later in the afternoon, but went on to recover from an unconvincing start to finish two under 69 to ensure his presence in the top 10, while former Open champion Louis Oosthuizen was forced to withdraw from the competition after suffering a suspected hamstring injury just eight holes into his game.

McIlory, who is still chasing his first win of what has been a disappointing year, had already dropped two bogeys before the 7th hole before a double bogey followed at the 379-yard 12th after his attempt at a greenside chip up the hill sailed well beyond the flag.

The Northern Irishman has been accused of lacking focus throughout the season, and if he is looking to silence those critics at Muirfield, McIlory will have to seriously improve on Thursday's showings. After sinking a birdie at the par three 13th, the 24-year-old's rotten display continued, sending a long putt at the 15th over the green directly into a bunker.

A second double bogey followed before McIlroy finished at the 18th. In a quaint conclusion, one of the Ulsterman's most vocal critics in Sir Nick Faldo also endured a disappointing day, finishing in identical fashion to McIlory at eight over from 79. Ian Poulter, who vented his frustration over the condition of the Murifield course after his session, also finished the day over par by one 72, with Ernie Els (three over 72) also failing to shine.

A crestfallen McIlroy told BBC Sport that he felt "unconscious" and brain dead" on the course after his uninspiring carding, admitting that he had struggled with the metal aspect of the game.

"It's not technique, it's not that I need to concentrate," he said. "But sometimes I feel like I'm walking around out there a bit unconscious.

"I've got to try to think a bit more, I'm trying to concentrate. I can't really fathom at the minute."