COLOGNE, Germany, June 22, 2012 (AFP) - The 27-year-old Chilean-born player, who was adopted by a Swedish couple when he was three months old, carded two eagles and four birdies in a second round 66 to reach 11 under par. 
That was two better than overnight joint-leader Fabrizio Zanotti, former Ryder Cup star Paul McGinley and English pair Danny Willett and Chris Wood.
"Now I saw that I'm up there, and of course it's going to be a new situation for me," said Sjoholm, the world 306.
"But then I watched a lot of Euro 2012 the last two or three weekends, I've seen what happens after four or five minutes, and I'm happy to be on top at the moment - but I would rather be on top on Sunday.
"I'm just enjoying this so much, I don't really think too much about it. It's just a great feeling and a fantastic tournament. I've always played here in Germany, and it was good to play well at the new venue."
Zanotti had shared the first-round lead with Marcus Fraser, but managed only a 71 while the Australian fell five behind with a 74.
Germany's Martin Kaymer, the highest-ranked player in the field, missed the cut, but 54-year-old compatriot Bernhard Langer shot 67 to be five under.
Leading and selected second round scores in the BMW International Open at Gut Larchenhof GC, Cologne, on Friday (GBR/IRL unless stated, par 72):
133 - Joel Sjoholm (SWE) 67-66
135 - Chris Wood 65-70, Fabrizio Zanotti (PAR) 64-71, Danny Willett 65-70, Paul McGinley 65-70
136 - Thomas Norret (DEN) 67-69, Shiv Kapur (IND) 66-70
137 - Joost Luiten (NED) 68-69, Niclas Fasth (SWE) 68-69, Matthew Nixon 71-66, Dylan Frittelli (RSA) 69-68, Oscar Floren (SWE) 67-70
138 - Michael Campbell (NZL) 70-68, Sergio Garcia (ESP) 71-67, Andrew Marshall 71-67, Rhys Davies 69-69, Simon Dyson 68-70, Marcus Fraser (AUS) 64-74, Richard Sterne (RSA) 69-69, Henrik Stenson (SWE) 70-68
139 - Robert Jan Derksen (NED) 69-70, Bernhard Langer (GER) 72-67, Christian Nilsson (SWE) 68-71, Steve Webster 69-70, Marcel Siem (GER) 68-71, David Lynn 69-70, David Drysdale 69-70, Thomas Bjorn (DEN) 69-70, Keith Horne (RSA) 66-73
140 - Soren Kjeldsen (DEN) 67-73, Ross Fisher 70-70, Richard McEvoy 69-71, Pelle Edberg (SWE) 66-74, Simon Khan 69-71, Pablo Larrazabal (ESP) 69-71, Wil Besseling (NED) 71-69, Gary Boyd 71-69, Stephen Gallacher 67-73, Richard Green (AUS) 67-73, Alastair Forsyth 69-71, Gonzalo Fdez-Castano (ESP) 71-69
Selected:
141 - John Daly (USA) 68-73, Rich Beem (USA) 71-70, Thongchai Jaidee (THA) 70-71
142 - Miguel Angel Jimenez (ESP) 68-74, Paul Lawrie 72-70
143 - Thomas Aiken (RSA) 72-71, James Kingston (RSA) 71-72, Colin Montgomerie 69-74
Missed cut:
144 - Jeev Milkha Singh (IND) 72-72, Hennie Otto (RSA) 69-75, Martin Kaymer (GER) 71-73, Shiv Shankar Prasad Chowrasia (IND) 75-69, Jose-Maria Olazabal (ESP) 72-72
145 - Darren Fichardt (RSA) 72-73, Jbe Kruger (RSA) 71-74
147 - Warren Abery (RSA) 74-73
148 - Matthew Zions (AUS) 73-75, Jeong Jin (KOR) 71-77
149 - Andrew Dodt (AUS) 74-75
150 - Alex Haindl (RSA) 75-75, Tjaart Van Der Walt (RSA) 70-80, Retief Goosen (RSA) 75-75
153 - Brett Rumford (AUS) 73-80
154 - Huang Wen-yi (CHN) 72-82