The American is one shot clear at the top of the leaderboard, despite this being the first time he has played at the Ohio course competitively.
"I knew it all through my caddie, who's been here so many years," the 27-year-old told reporters. "I just kind of had to listen to him.
"It's hard for us players to listen to our caddies, but he basically showed me where to go yesterday.
"I didn't feel like it was my first time because he has so much experience here."
Simpson is looking for his first win since last year's US Open at the Olympic Club.