The day's low round has Drewitt at 8-under par after opening with a 70, four shots clear of first round leader Daniel Bringolf and defending champion Brad Moules.
Sean Underwood, Geoff Drakeford and Ryan Peake are a further two shots back on 2-under-par.
Drewitt's course record betters the previous mark of 65 set by Brad Moules in last year's Classic at Glenelg.
"It was one of those rounds where everything went right," said Drewitt, who's ranked just outside the world's top-50.
"I drove the ball well and that gave me opportunities to attach the flags. The putts that I had were mostly insider five metres which enabled me to be aggressive with the putter."
Drewitt has yet to win a main amateur stroke title. He went close at Riversdale, but costly bogeys over closing holes damaged his chances and he missed the playoff with Matt Stieger and Nathan Holman by a stroke.
"I want to win a stroke event," says Drewitt.
"It's not something that I'm worried about achieving or anything like that. Just that it's something I feel that I need to achieve."
Drewitt this season has secured a top-16 at the NSW Amateur, a top-10 finish at the Lake Macquaire Amateur and a top-15 at the Master of the Amateurs.
With a tied 8th finish last week at Mandurah, Drewitt elected to compete in South Australia as opposed to defend his Queensland Amateur Championship title, an event which conflicts with the running of the South Australian Classic.
"I'm swinging the club the best that I have for a while," he says.
"I'll focus upon each shot at a time. That's all I can do, forget about yesterday's course record round, stay in the moment and try to post another low number."
The cut of 37 players came in at plus-8 with the 2011 Australian Amateur champion, Matt Stieger at 5-over-par, Nathan Holman at 4-over-par, Michael Williams and Kalem Richardson at 1-under-par, seven strokes behind Drewitt and all making the weekend in Adelaide.
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