Frost took advantage of unexpectedly benign seaside conditions for seven birdies without a dropped shot to finish one shot ahead of Louis Oosthuizen and Switzerland's Andre Bossert.
The top three players at the end of Friday's second round will gain automatic entry into the 134th Open Championship at St. Andrews.
Titch Moore and Omar Sandys shared fourth place at 4 under, with Scotland's Doug McGuigan. Zimbabwean Mike Lamb, Fulton Allem and Tjaart van der Walt tied for sixth one shot further back.
Last year's winner James Kingston and Grant Muller, who beat Bruce Vaughan in a playoff for the fourth and last spot a year ago, were both 1 under par with Tim Clark.
The 45-year-old Frost picked up where he left off at Leopard Creek in December last year, where he finished tied for sixth at the dunhill championship, though he had little reason to believe his form had stayed with him during a relaxing festive season.
'I played some social rounds and at the beginning of this week, I started hitting a few balls,' said the 1999 South African Open winner. 'I played a couple practice rounds here this week and that was my first look at the course.'
Oosthuizen, 24 years Frost's junior, knows the course only slightly better, having qualified successfully for the 2004 Open at Troon almost exactly 12 months ago.
'It was a little bit easier this time,' said Oosthuizen, who had nine birdies in his round. 'I had a little more confidence today than I did last year, it wasn't as nerve-wracking. It was a day to make low scores, but tomorrow it will probably be tough out there. I'll probably have to change my game plan.'
Frost, who finished tied for seventh place at the 1999 Open at Carnoustie, last played in The Open Championship at St. Andrews in 2000, but while in prime position to book a return to the home of golf tomorrow afternoon, he's taking nothing for granted.
'The course gave us a little bit today and the wind didn't blow as hard as it did in the practice rounds,' Frost said. 'If it works out that way, I'll be very pleased. I'm just going to try my hardest and take it one shot at a time and see what happens. I'd love to get in though, sure.'
Rd. 1 Scores
David Frost, Stellenbosch 65
Andre Bossert, Switzerland 66
Louis Oosthuizen, Pinnacle Point 66
Titch Moore, Walmer 68
Omar Sandys, Oppenheimer Park 68
Douglas McGuigan, Scotland 69
Michael Lamb, Zimbabwe 69
Fulton Allem, Orkney 69
Tjaart Van Der Walt, Durban 69
Mark Mouland, Wales 70
Grant Muller, Glenvista 71
Timothy Clark, Umkomaas 71
James Kingston, Rustenburg 71
Warrick Druian, Houghton 72
Jean Hugo, Unattached 72
Alan Michell, Clovelly 72
Andre Cruse, Johannesburg CC 72
Mark Murless, Blue Valley 73
David Ryan, Greytown 73
Tyrol Auret, Pollak Park 73
Andrew McLardy, Randpark 74
Michael Kirk, Royal Johannesburg 74
Henk Alberts, Pretoria 74
Barry Painting, Zimbabwe 74
Shaun Norris, Silver Lakes 75
Wayne Bradley, Modderfontein 76
Alan McLean, Atlantic Beach 76
David Faught, Zwartkop 76
Ian Hutchings, Glendower 78
Ryan Reid, Port Elizabeth 78
Bradford Vaughan, Macauvlei 79
Joakim Backstrom, Sweden 81
Thomas Aiken, Wanderers 82