Karlsson, who broke into red figures for the first time Friday with a birdie at the par-five third hole, has a 36-hole total of 8-under 136, one stroke better than first-round co-leader Jean-Francois Lucquin (71).
He birdied the par-five seventh for the second straight round, which led to a share of the top spot on the leaderboard, a position he did not relinquish.
"It's great to be sort of a little bit more under pressure," Karlsson said. "The most important thing is to keep playing the way I'm doing and keep building on all of the things I'm doing good."
Peter Hanson (69) is third at 6-under 138 and the other first-round co-leader, Richard Finch (73), is fourth at 5-under 139.
Defending champion Soren Hansen (67) shares fifth with Marcel Siem (68) at 4-under 140.
Pistorius testifies he didn't consciously pull trigger when he shot girlfriend
'Happy' fan videos make Pharrell cry [VIDEO]