Sergio Garcia's four-shot lead was cut to just one as he managed only a one-under 71 in the second round of the Nedbank Golf Challenge.
The in-form 2017 Masters champion stormed clear on Thursday as he looked set to dominate in Sun City, but his follow-up performance 24 hours later was less impressive and opened the door to challengers.
Home hopeful Louis Oosthuizen is just one shot back after Garcia dropped bogeys on three of the last six holes to reach 36 holes at nine under, having avoided any at all a day earlier.
A spectacular 18th where the ball circled the hole and rolled back out for a bogey summed up a frustrated round for Garcia.
Meanwhile, Oosthuizen impressed with a five-under 67 that included both a double-bogey and an eagle.
South African compatriot Branden Grace, who won the event last year, is in a seven-way tie for eighth in three under.
Rory McIlroy's week scarcely improved, however, following an even-par 72 with a one-under 71 that keeps him well off the pace ahead of the weekend.