Fisher, who played most of the 2005 season on the Challenge Tour, is through 36 holes at 8-under-par 138. Retief Goosen is five strokes back in second place after a second-round, 3-under 70 at Fancourt Golf Club.
Defending champion Tim Clark, who shared the overnight lead with Fisher, stumbled to a 5-over 78 and fell into a tie for ninth place at even-par. Clark carded just one birdie in his round.
Ernie Els, playing in just his third tournament since a four-month injury layoff and coming off a win last week at the dunhill championship, is among the five other players tied with Clark. The three-time champion followed up an opening-round 76 with a 3-under 70 at the par-73 layout.
Fisher was one of just a handful of players who successfully negotiated windy conditions Friday to post a good score. He carded a double bogey and two bogeys in his round, but added seven birdies to keep his score well into red numbers.
The 25-year-old began his round on the first tee and made the turn at 9 under after a spotless front nine that included a birdie at the second hole and three straight from the fifth.
Fisher's back nine included three blemishes -- bogeys at 11 and 15 and a double at 12 -- as well as three birdies, including two straight to end the round.
'I obviously wasn't happy with that double bogey on the 12th, but I'll say it was a 'solid' two-over if there ever was one,' said Fisher. 'I made my mistake but then I quickly corrected things.
'I love playing links courses and Fancourt is no different. You have to be so imaginative around the greens, especially since they are so firm. Any chip you hit has to be spot-on and you can't afford to go for the flag unless you strike the ball just perfect and put enough spin on it.'
Goosen, the 1995 champion, lost a shot at the second hole but went on to card two birdies over the next three holes. Three more birdies during a four-hole stretch on the back nine put him at minus-4, but a bogey at 16 dropped him five shots behind Fisher.
Els' 70 was good enough to move him up 43 spots from 52nd place at the end of the first round.
One stroke behind Goosen in a tie for third place at minus-2 are Leonard Loxton (71), Charl Schwartzel (74) and James Kingston (73).