The Spaniard, a two-time winner of the event, registered eight birdies in his bogey-free round at Gary Player Country Club.
South Africa’s Charl Schwartzel is one of three players on four under, along with Finn Mikko Korhonen and Frenchman Mike Lorenzo-Vera.
There is then a group of six at three under, including Schwartzel’s compatriot Louis Oosthuizen.
Englishmen Andy Sullivan and Matt Wallace, Australia’s Jason Scrivener, France’s Benjamin Hebert and China’s Wu Ashun are the others lying five shots back.
South Africa’s defending champion Branden Grace is at two under, along with New Zealand’s Ryan Fox, while Rory McIlroy, who went into this tournament still hopeful of winning a fourth Race to Dubai title, is a further two shots back.
The Northern Irishman made the fourth birdie of his round on the 13th before posting a second bogey on the 14th, and then double-bogeyed the 17th.
Garcia’s birdies came on the first, on four out of five holes from the seventh, and then on three of the last four.
The 38-year-old – going for a second consecutive win, having triumphed at the Andalucia Valderrama Masters last month – said in quotes on the European Tour website on Thursday: “It felt great. I rolled the ball nicely, I made some good putts here and there.
“It was one of those rounds where things happened nicely to me. I think I played very smart, I probably wasn’t swinging unbelievably but I kept it under control and kept the ball in play all the time which is difficult to do here.
“I managed to guess some of the winds here right and that is very, very difficult. When you can do that here, it’s nice to be able to shoot a score like this.”