The local favourite shot the lowest round of the day on Friday to head into the weekend alongside Crespi at 12-under overall in Johannesburg, one shot clear of nearest challenger Morten Orum Madsen.
Schwartzel's second round got off to the perfect start with birdies on the opening three holes before adding another two and an eagle to reach the turn seven under.
However, bogeys at 10, 13 and 17 cancelled a further three successive birdies on the back nine as the 29-year-old missed the chance to take an outright lead.
"It was just one of those nines where most things go right," he said about his front nine.
"I made some really good iron shots, put myself in play, and the greens are just in such fantastic condition that if you get the line right they go in, and that was pretty much the case.
"I holed some nice putts and played some great shots to turn in seven under. It looked like it could be a really low one at one stage.
"It's nice to turn in a low nine and get yourself quickly up the board. I got a bit derailed by a short putt on 10. Actually a good putt but, late on the afternoon on the greens with all the guys stepping round the hole, I managed to hit a little footprint and it came back at me. It sort of put the brakes on a little bit."
Crespi – playing in the first event of his maiden European Tour campaign – made an early charge to the top of the leaderboard as he followed up his 65 on Thursday with a 67.
The Italian hit four birdies on the front nine to reach the turn in 32 and, although he bogeyed 15, Crespi picked up two further shots to leave him happy at 12 under.
Denmark's Madsen, who shot 66, and South Africa's Christiaan Basson are one shot and two shots adrift of the leaders respectively with two rounds remaining.
Overnight leader Matthew Nixon started his day badly with a bogey at the first, but a birdie at eight got him to the turn in 36. A double bogey at 10 pushed him back before birdies at 13 and 15 ensured he ended the round eight-under par overall, four shots behind Crespi.
Jbe Kruger, who was level with Crespi after round one, continued his promising start into the second round with three birdies to ensure he is tied for fifth place.