Defending champion Kim Si-woo of South Korea is a shot behind on five under.
The veteran was 51-year-old Steve Stricker, who still can't decide whether to hang his hat in the big leagues or the senior circuit.
Johnson does not have a strong record at TPC Sawgrass, with a 12th-place finish a year ago his only top-25 showing in nine previous starts.
Starting on the back nine, Johnson sank five birdies in the space of seven holes and then moved to six under with another birdie on the second, before making pars for the rest of the round.
Johnson carded his lowest score - and first bogey-free round - at Sawgrass in 31 attempts, while closest rival and compatriot Thomas finished the day at one over par.
"The course is in immaculate shape, and it's going to firm up as the week goes on".
"And I knew that I could put the wedge in there just a little bit to the right of the pin and let it spin down".
Simpson, who has four top-10s against just one missed cut this season, briefly held the clubhouse lead after his 66.
On a day when the winds stayed down and there was little defense for the golf course, 38 players shot in the 60s. They're tough. I feel like they're tough to read. You can quickly make a big number. "I'm just never inside that range which I should be with the irons I'm having". He was at 7 under in the morning until a pair of bogeys late in his round dropped him to a 67. But it still requires you to hit the same kind of shots off the tees. "I've been playing well lately", Mitchell said.
"It's going to be punishment one of these days for sure".
"I think it's tomorrow and then in the weekend maybe greens will be more firm and faster. It will be challenging for sure, I'm well aware of that". Mitchell birdied his opening four holes, made seven birdies over his first 11 holes and was slowed only by a double bogey on the par-3 third. Rahm plays in the afternoon. "I used to be just too aggressive and it didn't work".
"I'm going to still go back to next week and try to get it back. (His) talent is unbelievable, so every time you play with him you look at something and learn something". "Took me a few years to learn that you didn't have to overpower it, not that I can overpower it anymore".
Mickelson blamed fatigue after the strain of fighting his way to a share of fifth in last week's Wells Fargo Championship.
"The only thing I don't like is holding up fingers", Johnson said. Justin Thomas is tied for 86th at 1 over while Rickie Fowler, the 2015 Players champion, is tied for 95th at 2 over.
Four players have a chance to surpass Johnson for No. 1 this week. "As it happened I hit my wedge straight at it and it took one bounce, checked and went in".
The best golf - and there was plenty of it on an ideal opening round for scoring - was all around them. He aggravated the injury on the driving range Wednesday when a golf cart drove in front of him, forcing Koepka to check his swing.
"If you're on your game, this course suits you", Kuchar said. Koepka added that the wrist was feeling better but not 100 percent.
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