Rory McIlroy was left shaking his head in frustration and disbelief after battling his way to an opening six-over-par 78 at the Memorial Tournament today.

The Northern Irish world number two had no real explanation for his struggles on a hot, humid day at Muirfield Village Golf Club but felt his overall game was in better shape than his score reflected.

"I'm pretty frustrated," McIlroy told reporters after mixing two birdies with six bogeys and a costly double at the par-three 12th where he four-putted from long range. "I'm trying not to let it get to me. It is what it is.

"A few bad rounds of golf isn't going to ruin anything, but I'd definitely like to start playing (better).

"I felt like it was really coming around at Charlotte and TPC, I started playing some good golf," he said, referring to the PGA Tour's Wells Fargo Championship and Players Championship where he recorded top-10s.

"And last week, Wentworth was bad conditions and stuff. It was sort of a weird week."

McIlroy missed the cut at the European Tour's flagship BMW Championship at a wet and cold Wentworth but was reasonably confident about his game ahead of this week's PGA Tour event hosted by Jack Nicklaus.

"I don't really have many explanations for this," said the 24-year-old.

"I felt like my game was good. I felt like I was coming in here and hitting the ball well. I felt like I needed to hole some more putts and things would be okay.

"I'm still sort of struggling on the greens a bit, and I just missed a few too many shots out to the right and that cost me. That's sort of been my big miss all of this year."

McIlroy has also been frustrated by his winless start to this year, having stormed to the top of the rankings in 2012 after triumphing four times on the U.S. circuit, including a second major title at the PGA Championship.

"Of course you want to come into this year and try and do the same sort of things," said McIlroy, who also won the money list titles in both Europe and the United States last season.

"It just hasn't really happened. I haven't lived up to my own expectations this year, but I'm working hard to try to live up to them, working hard to try and get the game back to where I know it can be."

McIlroy said he had not been affected by off-course distractions, having been widely criticised for changing his club manufacturer earlier this year, then refusing to confirm reports last week he had parted company with his management group, Horizon Sports.

"No," he replied, when asked if there was any correlation between his erratic golfing form and what had been happening to him outside the ropes.

"Once I'm here I'm focused on what I need to do. Right at the moment it's not happening for me. I don't think I've played as bad as what this score suggests. It was just one of those rounds that sort of got away from me."

McIlroy, a six-times winner on the PGA Tour who missed the cut at last year's Memorial Tournament, ended the opening round a distant 13 strokes behind early leader Charl Schwartzel of South Africa.