Two-time European Tour winner Finch lost his card at the end of the 2013 season and failed to regain it at the qualifying school, but received an invite to compete in South Africa.
A first round of 68 left the 36-year-old from Hull three shots off the overnight lead held by Denmark's Morten Orum Madsen, but that was overturned thanks to five birdies in his first 12 holes.
Finch, who won the New Zealand Open and Irish Open in 2008, picked up shots at the sixth, seventh and ninth to be out in 32 and also birdied the 10th and 12th.
That took him to nine under par and one shot ahead of Madsen, who was enduring a rollercoaster round as he looked to claim back-to-back wins following his triumph in the South African Open last week.
Starting from the 10th, Madsen had taken a double-bogey seven on the 13th but bounced back with three birdies in the next five holes and then eagled the par-five second from 15ft.
However, he then dropped shots at the next two holes to lie eight under par, two ahead of England's Ross Fisher and Portugal's Ricardo Santos.
Things went from bad to worse for Madsen when his tee shot on the fifth found the water guarding the green, leading to a double-bogey five.
And even though he bounced back with a birdie on the short par-four sixth, at seven under par he found himself three off the lead after Finch holed from close range on the par-five 15th for his sixth birdie of the day.
Fisher and Santos had also picked up another shot to join Madsen on seven under, while British Amateur champion Garrick Porteous had set the early clubhouse target on four under after a 69.