World number two Rory McIlroy felt "unconscious" and "brain dead" as he carded an eight-over-par 79 on day one of the Open at Muirfield.
The 24-year-old Northern Irishman is yet to win a tournament this season, after switching to Nike equipment in a multi-million dollar deal in January.
"It's not technique, it's not that I need to concentrate," said McIlroy, a two-time major winner.
"But sometimes I feel like I'm walking around out there a bit unconscious."
Mcllroy won his second major by a record eight shots at last summer's US PGA and topped the money list on both sides of the Atlantic to end the year as world number one.
But the 2011 US Open champion admits he is struggling with the mental side of his game.
McIlroy, who is tied 134th and 13 shots off the lead, pointed to a putt he hit across the 12th green on Thursday that went into a bunker on the other side on the way to a double-bogey six.
"I've got to try to think a bit more, I'm trying to concentrate. I can't really fathom it at the minute," he said.
"It's so brain dead. I feel like I've been walking around like that for the last month. It's being fully focused on each and every thing. It's something I've never experienced before."
McIlroy dropped shots at the fourth and fifth but made amends with a birdie at the short seventh to go out in 37 on a firm and fast-running Muirfield set-up.
He leaked two further bogeys immediately after the turn, but hit back from his double at 12 with a birdie at the short 13th. Another double bogey followed at 15 and he finished with two more bogeys to match the 79 he carded in the third round of the Masters in April.
"I wish I could say what's wrong or what I need to do to make it right," said McIlroy, who admitted he might need to consult a golf psychologist such as Dr Bob Rotella to address his current plight.
"I feel like I've got the shots, it's just matter of going through the right thought processes.
"I'm definitely under-thinking on the golf course. Maybe over-thinking about it off the course."
In the build-up to the Open, three-time winner Sir Nick Faldo urged McIlroy to focus on golf and avoid outside distractions, to which he replied: "Nick has to remember how hard this game can be at times."
But Faldo sought to clarify his comments on Thursday, saying he and McIlroy were friends and he was just giving some "grandfatherly" guidance not to take too much on.
McIlroy, who is in a relationship with former tennis number one Caroline Wozniacki, is in legal negotiations to leave Dublin-based management company Horizon.