Illuminated by a chip-in birdie at the par-5 tenth, the effort left the 30-year-old lying joint-sixth, just three shots off the lead in the $6.5m event at Sun City.
Knox, who has climbed into the world’s top 30 since winning the WGC-HSBC Champions in Shanghai last month, picked up birdies at the second and sixth to go out in 34.
He then moved to three under with that chip in, dropped his only shot of the day at the 11th before signing off in style with a birdie-3 at the last.
A good effort for his first competitive outing in South Africa, it saw Knox finish ahead of playing partners Bernd Weisberger (70) and Andy Sullivan (71).
South African Jaco Van Zyl and Sweden’s Henrik Stenson finished the day locked together in the lead on six-under-par after matching 66s.
Van Zyl, who gave such a good account of himself in last month’s Turkish Airlines Open, made his score by covering the last five holes at the Gary Player Country Club in three-under-par.
Stenson, the world No 7, stormed to the turn in five-under 31 then started for home with his sixth birdie of the day before parring in.
The pair sit a shot ahead of defending champion Danny Willett, with Australian Marc Leishman and South African Branden Grace both a stroke further back.
Earlier in the day, Scottish duo Scott Jamieson and Paul Shields both found the going tough in the first round of the Australian PGA Championship.
Jamieson signed for a seven-over 79 at the RACV Royal Pines Resort on the Gold Coast in Queensland.
Making his European Tour debut, Shields struggled to a ten-over 82 but still found himself sitting above Brandt Snedeker after the US Ryder Cup player slumped to a career-worst 84.
On a day when only six players broke par, South African Zander Lombard set the pace with a five-under-par 67 to lead by two from England’s Mark Foster.
On the Asian Tour, Thai-based Scot Simon Yates opened with a two-over-par 73 in the Ho Tram Open at The Bluffs in Vietnam.
Korea’s Charlie Wi set the pace there with a blistering nine-under 62 - four shots better than star attraction Sergio Garcia.
Elsewhere, former European Tour winner Raymond Russell got off to a promising start in the Beko Classic, which is being played over the Montgomerie Maxx Royal in Belek, Turkey.
Russell posted a one-under 71 to sit in a share of fourth, two shots behind leader Pontus Widegren from Sweden.
Fresh from winning the PGA Play-Offs, West Linton club pro Gareth Wright opened with a 73 - one less than defending champion John Daly.
Clydeway Golf’s Graham Fox, the third Scot in the field, found the going tough as he carded an eight-over 80.