The American world number two, who by his own admission has been playing some of the best golf of his life in recent weeks, carded a two-over-par 72 in the opening round at Firestone Country Club, a stroke worse than the day's average score.
Mickelson said he had lacked the necessary mental precision and lipped out from short range to bogey consecutive holes, the sixth and seventh, before ending the round in a tie for 41st place in the elite field of 73.
"I've been hitting the ball really well, and today I had a hard time focusing," the American left-hander told reporters after mixing three birdies with five bogeys on a Firestone layout softened by overnight rain.
"Mentally I wasn't sharp. I could tell I was a little bit tired or I just wasn't able to see the shot clearly. I wasn't able to see the putt roll clearly.
"I had trouble seeing the shot chipping, and I just had a hard time mentally visualizing. But I'm not concerned. I mean, my swing and game still feels very sharp."
Mickelson, who won his second PGA Tour title of the year by three shots in the British Open at Muirfield after closing with a brilliant five-under-par 66, aimed to come out guns blazing and refreshed for Friday's second round at Firestone.
"I just need to get a little bit of rest tonight and see if I can come out tomorrow a little bit more focused and start to see a little bit clearer what I'm trying to do," said the Californian, a 42-time winner on the PGA Tour.
"Like I said, I'm not overly concerned. I'm disappointed in the round. I certainly didn't want to shoot over par and leave myself this far back after the first day."
Mickelson trails the pacesetting Webb Simpson by eight shots heading into the second round.
"But my game feels sharp," Mickelson said. "I feel like I'm rolling the ball well. I know I'm swinging the ball well, and tomorrow if I can just mentally see it a little clearer, I should have a good day."
Asked to explain his lipouts for bogey from five feet at the par-four six and par-three seventh, he replied: "I hit them well, I rolled them well, right on my line, but the speed was bad.
"I just didn't quite get into the feel of the putt to be able to make it. I had that theme throughout the day on just about every shot."
Having already claimed five major titles in a Hall of Fame career, Mickelson is well aware of the danger of a possible mental let-down when competing soon after a big win.
"I am aware of it and I'll try to make sure I'm a little bit more rested and sharp heading into the weekend. But it does happen, it's a good problem to have," he smiled.
(This version of the story corrects reference to British Open from this month to last month.)
(Reporting by Mark Lamport-Stokes; Editing by Julian Linden)
