"I finished the round off, even though I didn't quite have it there at the end," Mickelson told PGATour radio. "I had a nice simple round going and then I missed the last four fairways.
"I was able to gather myself well and control my thoughts a little bit better."
Mickelson said after a first round 74 at last week's Memorial that his focus had been lacking but he subsequently gathered himself to tie for 13th.
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By playing in Memphis, the 47-year-old is continuing his traditional build-up for the U.S. Open.
A six-times U.S. Open runner-up, he has had three chances since winning the 2013 British Open to become the sixth player to complete a career grand slam, but he has not come close.
He even skipped last year's event - won by Brooks Koepka at Erin Hills - to attend his daughter's high school graduation.
Koepka matched Mickelson's 66 on Thursday, saved by a putter that was almost as hot as the Memphis weather.
"I didn't play well at all," he said. "We were all over the place. I'm a good putter. I just need to figure out everything else."
World number two Dustin Johnson also started well with a 67 that included a chip-in birdie at the par-four 12th.
After his approach shot ended in a hazard but dry, Johnson removed his right shoe and sock, took his stance in the mud and choked down on his wedge, before knocking the ball into the hole.
"I actually didn't hit it very well and got lucky and it went in the hole," he said modestly.
Leader Power, an Irishman who went to university in Tennessee, capped off his day by hitting his approach shot to five feet for a birdie at the last.
Power, 31, is in his second season on the PGA Tour.
(Reporting by Andrew Both in Cary, North Carolina, editing by Larry King / Ian Ransom)
