Campbell has been troubled by a chest ailment since May and despite extensive tests, no definitive diagnosis has been made.
"It was a bit scrappy out there," Campbell said of his round. "It really hurt warming up hitting driver so I didn't hit any drivers out there.
"I made a goal to get to 12-under for the week so that's still within sight and easily doable."
Campbell, who turned professional in 2012, said he wanted to win the event rather than just coast during the final two days to secure his playing rights.
"You need to keep the hammer down, it's a course where someone could easily get out and shoot eight-under."
Of the other Kiwis in the field, Daniel Pearce is in a share of 12th at two-under, Harry Bateman and Sven Puymbroeck are in a tie for 41st at three-over and amateur Josh Munn is in equal 46th at four-over.
Those with work to do in the final two rounds are: Mathew Perry (T54th, +6) and amateurs Vaughan McCall and Hayden Beard (T60th, +7) and Campbell Rawson (T73rd, +12).