SOUTH AFRICA 9 Jan, 2015 (AFP) - Sullivan, seeking a maiden European Tour, added a 70 to his opening 66 at Glendower Golf Club to reach eight under par, one shot ahead of Schwartzel.
However, four-time Major winner Ernie Els dropped off the pace with second round 77 after an impressive opening 67.
Sullivan, 27, who won a trip into space for a hole-in-one during the Dutch Open last year, carded four birdies and two bogeys in his round.
"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," said Sullivan.
"But anything in red figures (under par) 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."
Schwartzel, who has yet to win his national Open, finished birdie, eagle, birdie to record a 69 having considered pulling out of the event after fracturing a toe by walking into a piece of furniture.
"Any national Open should be the most difficult tournament and this week really is a good challenge," said the 2011 Masters champion.
"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."
Leading scores following the second round of the South African Open at the Glendower Golf Club on Friday (GBR/IRL unless stated, par 72):
136 - Andrew Sullivan 66 70
137 - Charl Schwartzel (RSA) 68 69
138 - Lasse Jensen (DEN) 67 71, Jj Senekal (RSA) 71 67, Colin Nel (RSA) 68 70
139 - John Parry 70 69, Alessandro Tadini (ITA) 71 68, Pablo Martin Benavides
(ESP) 71 68
140 - David Drysdale 68 72, Jared Harvey (RSA) 71 69, Rikard Karlberg (SWE) 70 70, James Morrison 71 69
141 - Thomas Aiken (RSA) 70 71, Matthew Nixon 72 69, Chris Swanepoel (RSA) 70 71, Matt Ford 70 71, Gary Stal (FRA) 75 66, Nacho Elvira (ESP) 70 71, Richard Sterne (RSA) 67 74, Matthew Fitzpatrick 73 68
142 - Niclas Fasth (SWE) 69 73, Branden Grace (RSA) 71 71, Paul Maddy 71 71
143 - Hennie Otto (RSA) 72 71, Anders Hansen (DEN) 73 70, Soren Hansen (DEN) 74 69, Estanislao Goya (ARG) 74 69, Ricardo Santos (POR) 72 71, Jake Roos (RSA) 73 70, Chris Paisley 74 69, Doug McGuigan (RSA) 73 70, Renato Paratore (ITA) 72 71, Tjaart Van Der Walt (RSA) 70 73
144 - Ernie Els (RSA) 67 77