The 26-year-old clearly likes home soil, having won this event and the Joburg Open in 2012 when he racked up his first – and last – wins with four on the European Tour in total.
Starting the day at Leopard Creek Country Club three ahead a 66, marred only by a bogey on the short fifth, saw him extend his cushion further as Italian Francesco Molinari was the only player to better his score, by one, and moved into second place on 11 under as a result.
Another South African Tjaart van der Walt shot 67 to move into third while last week’s Nedbank Golf Challenge winner Danny Willett, from Sheffield, continued his good form by adding a 69 to Thursday’s 66 to take a share of fourth.
“This is a golf course that suits me and if I keep hitting everything in play the way I have been the first two days then it should be a good weekend,” said Grace.
“I kept it going – there was a little bit of a slip there due to a lack of concentration but it happens.
“I’ve been hitting it well and kept on hitting good putts out there.”
Molinari began with five birdies and an eagle in 13 holes and missed another birdie chance at the next before dropping his only shot of the day at the 448-yard 17th.
“I holed some really good putts – the last four or five holes could have been better, but seven under is always a good round,” said the Italian.
“Branden has set a low target for everyone, so it was good to get some birdies.”
Former Ryder Cup captain Colin Montgomerie believes it would be “worth a punt” on Rory McIlroy winning six majors in a row by completing a 2015 clean sweep.
The Northern Irishman won the final two major championships of 2014 with victory at the Open in July and the US PGA Championship the following month, prompting Montgomerie to say: If you’re a betting man, I’m sure there are a lot of people putting money on it, I’m sure it’s a 100-1 shot but it’s worth a punt.
“If I was a gambler, which I’m not, I’d put money on that; he’s that dominant, and that good.”