Donald, aiming for the Number One spot on the Official World Golf Ranking, began the day a stroke clear of the pack, but things quickly turned sour with a double-bogey on the second.
He birdied the fifth and seventh to recover, but needed another salvage job - a birdie on the ninth - to rescue par at the turn after a bogey on the eighth.
The 33 year old is currently third in the world but, with leader Martin Kaymer having a week off and Lee Westwood playing in a tournament worth fewer ranking points in Indonesia, a win in South Carolina would move Donald to the head of the standings.
Things went much more smoothly on the way back, and a birdie on the 16th was enough to put him back in front.
"Obviously I got off to a pretty slow start, making seven on two," said Donald.
"I'm giving up two or three shots on that hole to the field.
"It wasn't really a terrible shot, I just pulled it slightly, it got a kick and if that ball stays a couple of yards inbounds it's an easy up-and-down for birdie.
"I dug deep. Made some good birdies. It was tougher today, the wind was more of a factor, the greens are getting firmer and crustier. And I was pleased with bringing it back after that slow start."
Defending champion Furyk suffered two bogeys - the second on the 18th - on the back nine to surrender the lead he had claimed from Donald earlier in the day.
Furyk birdied the third and fifth to join the Englishman at the top of the leaderboard. But although a bogey on the 12th was followed by birdies on the 13th and 15th, the final bogey of the day allowed Donald to nip back in front at 11 under for the tournament.
Brendon de Jonge enjoyed the best round of the day, signing for a 66 that moves him up to a tie for third alongside Scott Verplank, who went round in 67.