Hahn fired eight birdies and an eagle at East London Golf Club to finish 16 under par, two shots ahead of overnight leader Ricardo Santos, who added a 66 to his opening 62.
Ranked 881st in the world, Hahn started from the ninth with a birdie and picked up further shots at the 14th and 18th before a brilliant scoring burst took him to the top of the leaderboard.
The 24-year-old birdied the first and second, eagled the par-five third and then birdied the fourth, sixth and eighth to effectively cover the front nine in just 28 shots, one outside the European Tour record.
A 95-minute delay caused by torrential morning rain meant the second round was not completed on schedule on Friday evening, with play due to resume at 0715 local time on Saturday.
Former European Tour rookie of the year Santos, who had looked set to maintain his lead for most of the day after another flawless round, said: "I was very pleased to score so well today, I really couldn't ask for much more.
"I got quite lucky with the weather delay, because I was in some thick rough when we were called off, but the break let me breathe some clean air and when I came back out I saved par. There is still a lot of golf to come, but I'm very happy with my position at the moment."
England's Oliver Fisher is a shot behind Santos on 13 under, the 2011 Czech Open winner making birdies at his final four holes to shoot an error-free 63.
"It was a good day, I played well, hit the ball nicely and kept it in play," said Fisher, who lost his card after finishing 118th on the Race to Dubai last season. "There were lots of chances, which you can tell by the scoring. It was a matter of being patient and sticking with your round.
"I got off to a good start and then got called in on the fourth fairway. I made two birdies straight after that, so the delay helped me a little bit. I went dry for a while mid-round and then finished with four birdies, so it was a good day."
Hahn, who gained his European Tour card by coming through three stages of the qualifying school, said: "I just got the putter hot early and it stayed hot all day. It turned into a good one.
"I hit a lot of fairways and greens and the greens I missed early in the round I got up and down. I made a couple of putts you don't normally make and had a couple of unexpected ones go in."
Hahn admitted he had been inspired by the success of compatriots Peter Uihlein and Brooks Koepka, with Uihlein winning the Madeira Islands Open last year and Koepka earning promotion to the European Tour with three victories on the Challenge Tour.
"With Peter and Brooks coming over and watching the success those guys had, I knew playing mini tours in the US wasn't going to get me anywhere so this was a good place to try to accumulate world ranking points and move forward," Hahn added.
"It was a last-minute idea and it has turned into a really good opportunity for me. Seeing different countries and cultures is something I have never had. I am almost getting more enjoyment from that than the golf.
"Learning how to travel, eat different foods, it's kind of a growing up experience and I am having a blast every step of the way."