England's US Open champion Justin Rose said he struggled to focus during his first-round 75 at Muirfield.
Rose, seeking to become only the seventh man to win the US Open and Open Championship in the same year, is 10 behind leader Zach Johnson.
"It's very tricky out there, plays tricks with your mind," said Rose, 32.
"I have done my best to come here and be as prepared as possible, it was just a round that did not go my way. I am happy to have got it out of the way."
Rose, who became the first Englishman to win the US Open for 43 years at Merion last month, carded three bogeys, a double bogey and one birdie in tough conditions on day one.
"You have a shot of 180 yards and a nine iron in your hand and you're aiming 30 yards left and trying to bounce it 50 yards short," he added.
"I was not thinking about Merion so I can't use that as an excuse. I need to get a good round of golf together tomorrow and maybe be a bit more aggressive with my iron shots rather than trying to not make mistakes.
"I feel very comfortable as a major champion. I understand what works for me and how I can get the best out of myself, but it does not make the game any easier."