Fleetwood carded seven birdies and a solitary bogey at Harding Park to lie two shots behind clubhouse leader Li Haotong, who had earlier returned a bogey-free 65 to set the target on eight under par.
American Cameron Champ matched Fleetwood's 64 – the lowest score of the week so far – to finish five under alongside Brendon Todd and England's Paul Casey, who added a 67 to his opening 68.
Olympic champion Justin Rose was among the later starters after a four-under-par 66 on day one.
Jim Barnes is the only Englishman to win the US PGA, but the second of his two victories came more than 100 years ago in 1919.
Starting from the 10th, Fleetwood covered the back nine in 31 with four birdies and picked up another shot on the first before dropping his first shot of the day on the next.
The world number 13 bounced back with a birdie on the fourth and took advantage of the short par-four seventh to pick up another stroke before parring the difficult eighth and ninth.
"Yesterday I had a decent score going and had a bad couple of holes on 12 and 13 which halted progress," Fleetwood said. "Level par wasn't a bad score and today I feel like my game is improving day by day since I've come back.
"Today I hit a lot of fairways and that clearly makes it easier from there round this golf course."
Fleetwood is seeking his first major title after finishing runner-up to Brooks Koepka in the 2018 US Open at Shinnecock Hills and second behind Shane Lowry in last year's Open Championship at Royal Portrush.
"I'm the same person. I've got a little bit more experience," Fleetwood added. "If you look at it, was I playing better at Shinnecock? Maybe. But that might only be because I've only played two weeks since coming back.
"I feel the same, I guess, with a few more majors under my belt of experience. Hopefully they stand me in good stead over the weekend. Any time you're in
contention for a major, it's an experience, and I feel like I've done it a few times, so every time you learn.
"Every time these weeks come about you just hope – you prepare for it
to be your week, and today was a great day. Got a weekend of golf left. We'll see."
Li, who finished third in the 2017 Open Championship after a closing 63 at
Royal Birkdale, admitted his performance had come as a surprise after missing the cut in the Memorial Tournament and finishing 75th in the 78-man field in WGC-FedEx St Jude Invitational since returning to action.
"I've got no expectation actually because of staying at home doing nothing. I just want to go out there and have fun," the 25-year-old said.
"I didn't even think I could play like this this week, especially, like you said, got no confidence. Probably it helped me clear my mind a little bit this week."