While Drysdale tucked himself in just behind the leaders with a five-under-par 67, Russell was disqualified over an error on his scorecard.
Playing in his first full European Tour event in more than four years, the former Cannes Open champion shot a 71 only to be eliminated from the event after it was discovered that he had signed for 4s at the 13th and 14th when, in fact, he had taken a 3 and a 5 at those holes.
It was a devastating blow for Russell, who earned his place back on the main circuit after a good season on the Challenge Tour and had headed to South Africa straight from playing in the final stage of the Qualifying School in Spain.
Drysdale, who tied for eighth in last week's Alfred Dunhill Championship at Leopard Creek, resumed yesterday at two-under and picked up three more birdies in a flawless round.
Rookie Scott Jamieson was also on the leaderboard after getting to three-under, but a damaging double-bogey at the 16th – his seventh – was followed later on in the round by five successive dropped shots as he carded a 72.
Defending champion Richie Ramsay's 71 was matched by Peter Whiteford and Lloyd Saltman, who recovered well from dropped shots at his opening two holes, with George Murray a further shot adrift.
Alan McLean signed for a 75, one less than Elliot Saltman, who had a triple-bogey 7 at the 16th on his card.