Padraig Harrington, second on his own before he double-bogeyed the final hole of his first round, was in a tie for sixth spot on five under after 11 holes.
Grace rejoined Aiken with a birdie on the tenth, but Colsaerts's misery continued with a fifth dropped shot of the day two holes later.
Into third place came Slattery, who birdied the last for a superb 65, and also Harrington, who packed five birdies into eight holes from the seventh to be only one behind as well.
What made Slattery's day even better was that playing partner Goosen had shot 68 and their English amateur Mark Vandenberghe had also helped the two pros to win cars worth £25,000.
For the second day running Harrington finished with a double bogey seven - and having bogeyed the previous two this time he had fallen all the way back to four under.
That was eight behind Grace, who went three clear of the field with a hat-trick of birdies from the 14th.
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