Coming off a second-place tie in Connecticut after also opening with a 62, Poston eagled the par-5 second and had seven birdies in a bogey-free round at TPC Deere Run.
"It's, obviously, why you practise and why you work as hard as you do on your game," Poston said.
"Any time you can have those rounds where you really get it going low and mistake-free, bogey-free is a bonus. It's why you work at it and why you practise the way we do."
Canadian Michael Gligic was second at 64, playing bogey-free in the final group of the day off the first tee.
"It was nice," Gligic said. "Just kind of hit the ball where I was looking. Gave myself lots of looks. I think I only missed one green and was able to make a few putts."
Vaughn Taylor and Christopher Gotterup followed at 65. Ricky Barnes, Denny McCarthy, Chris Naegel and Dylan Frittelli shot 66.
"Been struggling a little bit physically and haven't been putting very well," Taylor said. "But coming into here I told myself, 'You know, I know this course, how to play it. Just go do it.'
"That's kind of how I did today. Just tried not to think about too much and just play golf."
Poston won the 2019 Wyndham Championship for his lone PGA Tour title.
"I would say for the last few months it's been in a good spot," Poston said. "Just hadn't quite seen the results.
"Then, Hilton Head I had a good week. Wells Fargo, good week. Last week, playing well. So I think it's starting to kind of come together and see the results and see the shots and kind of building that confidence back into my ball- striking.
Iowa native Zach Johnson, the 2012 winner, opened with a 69 in afternoon wind in his 20th consecutive start in the event.
"That's as hard as I've seen this golf course play in a long time," Johnson said. "Granted, you got some roll, so if you land it in the fairway it was going to bounce, which is fun. But it was hard."
Defending champion Lucas Glover shot 74.
Cam Davis is the best of the Australians. His three-under 68 sees him tied 19th, one ahead of compatriot Aaron Baddeley.
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