Hensby is a shot adrift of New Zealander Steve Alker who set the early pace with a stellar seven-under 63 on Thursday at Sunningdale in Berkshire, England.
Hensby's 64 featured five-straight birdies to start - he landed two more, but also a bogey, in his six-under round.
"The first three holes, I wasn't outside three feet," the 54-year-old said.
"The back nine probably is a little bit more difficult ... there's a couple of tricky tee shots."
Last month, Hensby was tied with Padraig Harrington and Stewart Cink going into the final round of the US Senior Open before finishing fourth, an experience he planned to draw upon in coming days.
"I learned a lot playing with Padraig and Stewart the last day," he said.
"You don't have to be perfect and, unfortunately, we feel like we have to.
"Padraig played great the last nine holes and it was fun to be there and watch and competing, even though I wasn't in contention the last nine holes."
Hensby, whose only senior tour victory is the 2023 Invited Celebrity Classic, trails Alker who had no issues with the back nine, shooting five-under 30 in the stretch.
American Joe Durant and South Korea's KJ Choi, the defending champion, are tied for third at four-under with a host of players a further shot back, including Harrington and Ernie Els.
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