Sullivan added a solid two-under 70 to his opening 66 at the Glendower Golf Club in Johannesburg to set the early clubhouse target at eight under, which proved enough to earn the outright lead at the halfway stage.
The Englishman, who posted four top-fives on the European Tour last season and finished 33rd in the Race to Dubai, started at the 10th and reached nine under after birdies at the 11th, 12th and 17th.
But he was hampered by an ???errant??? driver over the front nine and dropped shots at five and seven before getting back on track with his fourth birdie of the round at the eighth.
“I thought it was going to be a really good knock on the front nine, but a few little errant drives meant the rough got its payback on me today,??? he told European Tour Radio.
“But anything in red figures is good so I am really happy with the position I am in. I thought I did well on eight to make birdie after a couple of smelly holes in the middle.
“When I get in these positions I find myself enjoying it more. It’s where you want to be, you practice to be in these situations and I am playing with the guys I always wanted to as a kid. It’s fantastic.
“It’s new territory to be up there after two rounds but hopefully I can take what I usually do in rounds three and four and blow the field away.”
Sullivan???s closest challenger is former Masters champion Charl Schwartzel, who ignited his bid for his first national title with a superb birdie-eagle-birdie finish after struggling for much of the day.
Schwartzel was one over for his round through 15 holes, but his late flourish capped a three-under 69 and lifted him into outright second ahead of Denmark???s Lasse Jensen and South African pair Colin Nel and JJ Senekal.
“Any national open should be the most difficult tournament and this week really is a good challenge,” said Schwartzel, who almost pulled out of the tournament after suffering a fractured toe in an accident at his home.
He added: “The only reason I have not pulled out is that I can actually hit the ball without pain. For some reason I can turn onto the foot but the walking is a challenge.
“I was struggling for a whole bunch of last year but started working with my dad again to recreate the feelings I had and get the old basics back. It’s starting to get more comfortable and I’m at least hitting shots that I’m seeing.
“I got off to a really iffy start this morning, missed a few short putts and the conditions got difficult, but I just figured if I could somehow get to level par and try again tomorrow, and I was pretty much on track until a nice finish at the end.”
Former amateur world No 1 Matthew Fitzpatrick recovered from an early double-bogey with four birdies and an eagle in a 68 which lifted him to three under, but Els slipped eight shots adrift of Sullivan after starting the day only a shot behind.
The tournament host and ambassador was erratic from the start as he offset two birdies with three bogeys over the first eight holes before it all went horribly wrong at the ninth.
A wayward approach found water as he ran up a triple-bogey seven to turn in 40, and he also took seven shots to negotiate the 10th after he needed to attempts to escape a fairway bunker and compounded his errors by three-putting.
But as he flirted with the halfway cut, Els regained his composure and restored his spirits with birdies at the 13th and 14th as he salvaged a five-over 77.