The winner of the elite, 12-player event will earn $2 million, the richest first prize in golf.
Els, playing in his home country, had an eagle on the 17th hole and is at 135. DiMarco is next after a second straight 68. Darren Clarke rebounded with a 67 and is third at 139.
Retief Goosen and Jim Furyk are at 140, with Nick Price and Robert Allenby at 141. At 142 are Michael Campbell, Padraig Harrington and Bob Estes. Sergio Garcia and Colin Montgomerie complete the field at 143.
Furyk led briefly during the opening holes of the second round, but he shot a double-bogey 6 on the 11th and had to settle for a 71.
DiMarco, winner of the Phoenix Open, is returning to South Africa after playing on a local tour as a rookie 10 years ago.
``It's a lot better playing for $2 million than for a total purse of a 10th of that,'' he said.
Els' eagle-2 came on the 17th when he hit a 9-iron 165 yards into the hole. He had a birdie the hole before.
``That chip-in for eagle has to be the shot of the year for me,'' said Els, who won he British Open this year. ``I don't think I have ever played two holes in four shots.''
Montgomerie regrouped after an opening 74 with a 69 that came despite a double-bogey 7 on the ninth hole.
Price, who has won the tournament three times, has been struggling with his putting. He has had rounds of 70 and 71.
``If I can get some 15- to 20-foot putts to go down over the weekend, I'll be in great shape,'' he said.
Full-field scores from the Nedbank Golf Challenge