An eagle-birdie finish gave the 32-year-old a three-shot lead at the Agadir resort as he seeks to add to his two PGA European Tour wins, the more recent of which came last July in France.
A strong result would also give the world No 72 a chance of breaking into the top 50 ahead of the qualification cut-off for Augusta.
A bogey at the second was an inauspicious start to his round yesterday, but he responded with an eagle and two birdies and picked up further shots at the tenth and 11th before chipping in at the last two holes.
He said: “Sometimes you need a bit of luck in this game and thankfully today was one of those days where it went my way. 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 here.
“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 was Siem’s closest challenger, although he failed to make the most of a strong start. Teeing off at the tenth, Velasco birdied his first three holes, but could only pick up two more shots for a five-under 67.
England’s David Horsey, the 2011 champion, was four-under alongside compatriots Graeme Storm and Simon Wakefield. Another Englishman, Gary Lockerbie, and Scot Craig Lee were three-under, with France’s Gregory Havret and Dutchman Joost Luiten a further shot behind.
The other players playing for Augusta invites struggled on a windswept day, with Spain’s Rafa Cabrera-Bello and Scotland’s Richie Ramsay two over par after rounds of 74.
Ramsay’s two compatriots, Stephen Gallacher and Scott Jamieson, were even further off the pace at three over and five over respectively.
Two recent winners of the event were well back in the field, with defending champion Michael Hoey also on two over and Rhys Davies, victorious in 2010, only one shot better off.
But nobody could match the struggles of Czech player Jakub Svoboda, who was well on course for a European Tour record score when he retired after hitting 21 over par from just six holes.
Svoboda withdrew with a wrist injury after going two over par on the first, then dropping four shots at the second and five at the third where he had his first nine of the round.
Three dropped shots on the fourth, two on the fifth and five on the par-four sixth saw him bring his round and tournament to a premature end.
Welshman Philip Price was another player to pull out, due to a cracked tooth, but completed a round of 79.
Fading light saw play suspended with the final group still playing their last hole, the ninth.