The American, who was in euphoric mood after winning his fifth major title in stunning style at Muirfield in Scotland two weeks ago, mixed three bogeys with two birdies to finish in a tie for 21st at one-over 281.
Though delighted to be back in golf mode, Mickelson felt he had suffered from a post-British Open whirlwind of victory celebrations, media commitments, travel and a visit to Oak Hill Country Club, venue for next week's PGA Championship.
"I didn't have a chance to prepare properly," said the Californian, a 42-time champion on the PGA Tour who finished a distant 16 strokes behind runaway winner Tiger Woods at Firestone Country Club on a warm, breezy Sunday.
"I was doing some other ancillary stuff (before the Bridgestone Invitational). But I had a good week of practice. My game does feel pretty good, but I'll have to be a lot sharper on each shot next week."
Mickelson has a curious record at the Bridgestone Invitational, producing top-10 finishes in his first four appearances in the elite World Golf Championships (WGC) event but only one in his last 11.
"I've always enjoyed playing here," the world number two told reporters about the heavily tree-lined Firestone layout. "Early in my career I've had a lot of success, and late in my career I've not had success here.
"But it has not changed the way I feel about the course and the tournament. It's just a wonderful straightforward test of golf. The golf course was in great shape this week."
Asked to explain his recent troubles at Firestone, he replied: "I don't have a great reason as to why I haven't played well the last eight, 10 years, but I wasn't as sharp this week as I needed to be, as I need to be next week."
Having played a practice round at Oak Hill in Rochester, New York six days ago, Mickelson has a clear-cut strategy mapped out as he prepares for the August 8-11 PGA Championship.
"I don't have to do too much course work," said the left-hander, who won the 2005 PGA Championship at Baltusrol. "I've studied the golf course. I know exactly how I'm going to play it.
"I just need to get my game sharp now. I'll probably take tomorrow off and rest. And then I'll play Tuesday a practice round, and Wednesday will depend on how I feel at the time."
(Reporting by Mark Lamport-Stokes; Editing by Julian Linden)
