AGADIR, Morocco, March 28, 2013 (AFP) - Siem, twice a winner on the European Tour, carded an eight under par, a round topped off by an eagle-birdie finish as the world number 72 eyed the top-50 ranking he needs to make the Augusta qualifying mark.
"I didn't know I still had a chance for the Masters but now we know there are four or five of us in the field here who still might be able to get to the Masters if we win," said Siem.
"It is a nice incentive but I came here first and foremost to play this tournament and play this golf course. The Masters would be pretty special but I just have to concentrate on what I am doing in this tournament."
Spaniard Alvaro Velasco is second overnight after a five under 67 as both he and Siem enjoyed the better afternoon playing conditions at the Golf du Palais Royal.
Windy conditions in the morning meant that only four players broke par -- that group included 2011 champion, England's David Horsey who shot six birdies and two bogeys.
At the other end of the scoreboard, the Czech Republic's Jakub Svoboda was 21 over after just six holes before he pulled out with the European Tour's worst score beckoning.
The 20-year-old, ranked 1,522 in the world, carded a seven, seven, a nine, another couple of sevens and a nine at the sixth.
The European Tour record high score for a single round is the 111 by Mark James at the Italian Open in 1978.
Leading and selected first round scores in the European Tour Trophee Hassan II at Golf du Palais Royal on Thursday (GBR/IRL unless stated; par 72):
64 - Marcel Siem (GER)
67 - Alvaro Velasco (ESP)
68 - David Horsey, Simon Wakefield, Graeme Storm
69 - Craig Lee, Gary Lockerbie, Mikko Ilonen (FIN)
70 - Joost Luiten (NED), Gregory Havret (FRA)
71 - Paul Waring, David Howell, Scott Arnold (AUS), Richard McEvoy, Julien Quesne (FRA), Kang Kyung-nam (KOR), Justin Walters (RSA), Andreas Harto (DEN)
72 - Daniel Popovic (AUS), Jorge Campillo (ESP), Matthew Baldwin, Scott Henry, Matthew Southgate, Moritz Lampert (GER), Pablo Larrazabal (ESP), Jose Manuel Lara (ESP), Alessandro Tadini (ITA), Bernd Wiesberger (AUT), Chris Lloyd
73 - Francesco Molinari (ITA), Mikael Lundberg (SWE), John Parry, Garth Mulroy (RSA), Rhys Davies, Chris Paisley, Oliver Fisher, Brett Rumford (AUS), Joachim B Hansen (DEN), Robert Rock, Andrew Marshall, Ewan Porter (AUS), Eduardo De La Riva (ESP), Alejandro Canizares (ESP), David Higgins, Bernd Ritthammer (GER), Alexander Levy (FRA)
Selected:
75 - Daniel Gaunt (AUS), Richard Green (AUS)
77 - Keith Horne (RSA)
78 - Matthew Zions (AUS), Tjaart Van Der Walt (RSA)
84 - Carl Yuan (CHN)