The 33-year-old American is in good company at the top of the leaderboard as Aussie Marc Leishman, American Kyle Stanley, Jon Rahm of Spain and reigning Masters champion Sergio Garcia share second after shooting 67s.
The world No 1 also birdied 13 and 15 either side of a rare lapse on the 14th green as he three-putted from 30 feet, and he then drilled a sublime 220-yard six-iron to four feet at the 18th to set up an eagle which took him to four under at the turn. His 66 marked his lowest opening round at TPC Boston.
World number one Johnson picked up where he left off when he won Northern Trust tournament on Sunday as he is off to another roaring start in the second event of the FedExCup Playoffs.
"I've been lucky enough to win at a couple of courses multiple times in my PGA Tour career, and I think just mentally you're just a lot more comfortable. I feel like the things I've been working on, they are back to working". So the confidence was just down a little bit. But after last week, obviously I've got a lot of confidence now. "I've got a lot of control over it and I feel like I can control my golf ball very well, which is what you've got to do in these conditions".
"It would just be nice to get a win before I shut it down for a while". And the ones that I didn't make, they looked like they were going to go in. "So a lot of positives". "The Australian Open holds a special spot in my heart", Day said.
"I wasn't seeing the shots that I wanted to see". That was even with a short 4'5 miss early on in his round. He shot a 66 at Quicken Loans National. "That's the thing - once you know you can make birdies on a golf course, it makes it easier to do that again".
Kevin Na, Luke List, Brian Harman, Jason Dufner, Jonas Blixt and Grayson Murray are a shot further back, a stroke ahead of the likes of Rickie Fowler and Phil Mickelson.