In all, the victor at Sun City last November recorded seven birdies and one bogey and before the turn looked to be slipping out of contention.
Grace, a South African, had a haywire front nine that included two eagles, two birdies and two bogeys.
"I didn't know, I have a joint course record but it's quite cool".
"I can't wait for the weekend", said Paisley, who won the 2012 English Challenge on the European Challenge Tour.
Saddier joined Paisley at the top of the leaderboard after the Englishman earlier followed up a first-round 66 with a near flawless 65 in cloudy conditions.
"Through 13 holes, I think I was five or six behind".
"I can not wait for the weekend", said the Englishman.
"Yeah, obviously being tied for the lead isn't a position I've been in a lot", Paisley said. "I really had no expectations whether I was going to play good or bad after having some time off".
"At the moment it is weird - I feel like I did not play wonderful golf".
Defending champion Graeme Storm of England, who pipped golf superstar Rory McIlroy of Northern Ireland in a playoff past year to win the second-oldest national championship after The Open, carded a 75.
"I am very happy, I just wish I had more holes to play because I feel I could have birdies every hole today", said the 2017 Joburg Open champion, who was also frustrated with a slow start.
Young South African Jacques Kruyswijk is their nearest challenger on nine-under-par after a 67 in what is his rookie season on the European Tour.
South African Retief Goosen, one of the former Major winners, was seven under for the day as he teed off on 18, only to suffer a double-bogey six that left him five strokes off the pace. After making seven and eight birdies in his first and second rounds respectively, he managed just three on Saturday and then dropped a shot at the par-four 16th hole - his only blemish on the day and only his third bogey all week.
English duo Andy Sullivan and Matt Wallace carded three-under 69s to move to five under for the championship while former Ryder Cup victor Jamie Donaldson, who carded the first ace of 2018, is a further two shots back after a one-under 71 which saw the Welshman mix five birdies with four bogeys.
Paisley, who shared a four-shot overnight lead with Adrien Saddier of France, carded a two-under 70 over the 6,942-metre/7.594-yard layout 12 kilometres (7.5 miles) east of Johannesburg.
Former Major title-holders Charl Schwartzel (139) and Ernie Els (141) of South Africa made the cut for the final two rounds.