AGADIR, Morocco, March 30, 2013 (AFP) - The 32-year-old Siem, who needs to win on Sunday to climb into the world's top 50 and earn a ticket to Augusta, finished the day on 15 under par, a round boosted by four birdies in six holes around the turn at the Golf du Palais Royal.
Spain's Pablo Larrazabal, 2011 champion David Horsey of England and Finland's Mikko Ilonen are all grouped in second place.
After Siem had bogeyed the first, Horsey and Ilonen joined the German in a share of the lead.
But once the leader had hit two long birdie putts at the seventh and ninth to turn one ahead, his rivals struggled to stay in touch.
Siem extended his lead with a birdie at the long tenth before striking his tee shot to within a foot of the 12th pin.
"I had to hole a few big putts just to stay ahead and then I managed to pull away a little bit," said Siem.
"Fifteen and 17 were disappointing because you are really looking to pick up a couple of shots there but I managed to give one away.
"There was pressure there so it was great to make a few putts and pick up four in six holes. It is very nice to have the gap back for the start of the final round tomorrow.
"I am pretty confident at the moment so I know the swing is there when I trust it and just let it go. That will be very important."
Leading and selected third round scores in the European Tour Trophee Hassan II at Golf du Palais Royal on Saturday (GBR/IRL unless stated; par 72):
201 - Marcel Siem (GER) 64-68-69
205 - Mikko Ilonen (FIN) 69-66-70, David Horsey 68-67-70, Pablo Larrazabal (ESP) 72-64-69
207 - Bernd Wiesberger (AUT) 72-66-69
208 - Craig Lee 69-69-70
210 - Mikael Lundberg (SWE) 73-69-68, Alvaro Velasco (ESP) 67-74-69, Matthew Baldwin 72-70-68
211 - Joost Luiten (NED) 70-69-72, Andreas Harto (DEN) 71-67-73, Simon Wakefield 68-72-71
212 - Richie Ramsay 74-69-69, Scott Arnold (AUS) 71-71-70, Oliver Fisher 73-69-70
213 - Emiliano Grillo (ARG) 74-70-69, David Drysdale 74-72-67, Stephen Gallacher 75-68-70, David Howell 71-70-72
214 - Francesco Molinari (ITA) 73-71-70, Gary Lockerbie 69-76-69, Garth Mulroy (RSA) 73-67-74, Paul Waring 71-72-71, Richard Green (AUS) 75-71-68, Julien Quesne (FRA) 71-72-71, Anthony Snobeck (FRA) 75-67-72
Selected
215 - Daniel Popovic (AUS) 72-72-71
218 - Keith Horne (RSA) 77-69-72
221 - Justin Walters (RSA) 71-71-79