Swede Alexander Bjork and Scot Scott Jamieson shared the Tshwane Open second-round lead Friday, but South African Thomas Aiken stole the show with a nine-under 62.
Aiken carded halves of 31, and at one stage the course record of 60 appeared under threat as the three-time European Tour winner unleashed a birdie blitz.
On the front nine he picked up five and dropped one shot, and grabbed five more birdies on the back nine thanks largely to hot putting.
The South African, who began with a 72 on the 6,476-metre (7,081-yard) parkland course, is two shots off the pace with compatriots Peter Karmis and Justin Walters.
Bjork, one of three leaders on 65 after the opening round at Pretoria Country Club, added a 67 for a halfway total of 132 in the European Tour event.
Jamieson followed up an opening 67 with a 65 as he seeks a second title having won the Nelson Mandela Challenge in South Africa five years ago.
Sandwiched between the leaders and the South African trio is Englishman James Morrison after rounds of 67 and 66 on a course that became tougher in the afternoon as a breeze gathered strength.
Those on 135, three strokes behind the co-leaders, include Frenchman Gregory Havret after a second-round 70.
Seeking a first European Tour success in nine seasons, Havret would have been closer to the top of the leaderboard but for a double-bogey six at 15.
Finding a greenside bunker with his approach shot on a par-four, he took two strokes to get out of the sand, then two putted.
Although nine shots off the pace, one the happiest golfers after the second round was former US Masters champion Trevor Immelman.
Little has gone right for the South African since his triumph at Augusta, but he finally ended a disastrous run of missing every cut since last May.
A three birdie-two bogey round of 70 gave him a halfway total of 141, and the welcome chance to compete in the final two rounds.
Former British Open champion Darren Clarke of Northern Ireland — the other major winner in the field — was less successful and a bogey-riddled 77 condemned him to an early departure from Pretoria.
A midday thunderstorm halted play for 57 minutes, but the Tshwane Open has so far been spared the torrential rain that reduced the Joburg Open to three rounds last weekend.
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