(CNN) -- Unconscious. Brain dead. Rory McIlroy was in a brutally reflective mood in the words he chose to describe his opening round in the British Open
The world no.2 carded an eight-over-par round of 79 as the world's golfing giants made their first forays on Scotland's testing Muirfield course.
All the golfers are contending with the rarities of a British heatwave but McIlroy said he was tortured by a long-term torpor.
"Sometimes I feel like I'm walking around out there and I'm unconscious," said McIlroy, a two-time major winner, told the media.
"I just need to try to think more. I'm trying to focus and trying to concentrate.
"But I can't really fathom it at the minute, and it's hard to stand up here and tell you what's really wrong
"It's just so brain dead. Seriously, I feel like I've been walking around out there like that for the last couple of months. I'm trying to get out of it."
McIlroy lost his top ranking to Tiger Woods in March and the Northern Ireland golfer is still searching for his first win in 2013, having only chosen to enter seven tournaments before the Open.
The 24-year-old's slump in form has also been linked to his switch to Nike equipment this year in a deal reported to be worth $250m.
McIlroy struggled to the turn at Muirfield in one over before two painful double-bogeys and four bogeys on the back nine left him searching for answers.
A comeback of epic proportions is required if McIlroy can make the cut for the third major of the year.
"I want to try to be here for the weekend," McIlroy said. "But the thing that I need to do tomorrow is just go out there and freewheel it and try and make birdies and try and play with that little bit of whatever it is I have usually."
McIlroy faced questions about his dubious form on the eve of the Open with suggestions his high-profile relationships with Danish tennis player Caroline Wozniacki was a distraction.
But he rebuffed that suggestion, saying: "No, not at all, no, no. It's not that at all. No, I'm fully focused on the golf out there."
McIlroy wasn't the only golfer mired at Muirfield, where temperatures nudging 80 degrees Fahrenheit (26 Celsius) had hardened the course and sped up the greens.
U.S Open winner Justin Rose finished on four over par after carding a 75 which included just a single birdie on the 17th hole.
Fellow Briton Luke Donald, the world No. 9, finished five shots further adrift of Rose after going round the links course in 80 while Australian Masters champion Adam Scott settled for a par-round of 71.
Tiger Woods, on the hunt for his first major win since 2008, was one over at the turn but clawed his way back into contention on the back nine.
The 14-time major winner lashed his opening tee-shot into the rough and ended up taking a penalty drop but four birdies - and just one bogey - on the run-in helped him finish on two-under.
"There wasn't a lot of talking out there because we're trying to grind it out on that golf course, and it's one of those courses where it just got so difficult," Woods told reporters.
"As the golf course dried out it got quick. Some of these putts today, I mean, I putted the ball off the green today. And it really wasn't that bad a putt."
Fellow American Zach Johnson, winner of the 2007 Masters, held the overnight lead with a five-under 66.
It was a punchy round from the 37-year-old, who eagled the long par-five fifth and dropped just a single stroke at the 14th.
"Any time you shoot under par in an Open or a major, for that matter, you have to be putting at least decent and I putted great," Johnson told reporters.
The American holds a one-shot lead over 57-year-old compatriot Mark O'Meara and Spain's Rafael Cabrera-Bello, who both carded 67.
Next came a group of five players -- Americans Dustin Johnson, Brandt Snedeker and Tom Lehman, Miguel Angel Jimenez of Spain and India's Shiv Kapur -- on three under.
American teenager Jordan Speith, who beat Zach Johnson and David Hearn to the John Deere Classic title in a play-off earlier this month, joined Woods on two-under.
That victory earned the 19-year-old the last spot in the British Open and the debutant is in relaxed mood at Muirfield.
"It has been a great couple of weeks," Speith said. "I've kind of accomplished more than I'd thought possible this year."
What wouldn't McIlroy give to be able to say the same thing?