AGADIR, Morocco, March 31, 2013 (AFP) - Siem captured the European Tour's Hassan 11 Trophy in Morocco on Sunday and had moved to 49th in the projected world rankings list after his third Race to Dubai victory.
But Sweden's Henrik Stenson clinched a strong finish at the storm-interrupted US PGA event in Houston by seizing the clubhouse final-round lead on 15-under 273, and was assured of jumping from 53rd in the world rankings and ahead of Siem.
The German will not crack the top 50 in the world rankings on Monday to book a Masters spot.
American Russell Hendley, who only needed a top-53 showing in Houston to reach 50th in the world, was sharing 45th after a final-round 68, meaning he would jump to 50th and bump Siem to 51st.
Siem, the defending French Open champion, started the day four clear but bogeyed the first and was caught when Finland's Mikko Ilonen, who had suffered food poisoning overnight, had a hat-trick of gains from the second.
Siem responded with birdies at the fifth and seventh, but Ilonen matched it to remain within touching distance.
The German then birdied the 11th from ten feet as Ilonen found sand from the tee at the 12th.
Siem birdied the 15 before going on to card a final round two under par 70 and 17 under for the tournament.
Ilonen had two closing birdies and shared second place with 2011 winner David Horsey of England.
Leading and selected final round scores in the European Tour Trophee Hassan II at Golf du Palais Royal on Sunday (GBR/IRL unless stated; par 72):
271 - Marcel Siem (GER) 64-68-69-70
274 - Mikko Ilonen (FIN) 69-66-70-69, David Horsey 68-67-70-69
276 - Pablo Larrazabal (ESP) 72-64-69-71
279 - Alvaro Velasco (ESP) 67-74-69-69, Matthew Baldwin 72-70-68-69, Bernd Wiesberger (AUT) 72-66-69-72
281 - David Howell 71-70-72-68
282 - Craig Lee 69-69-70-74, Julien Quesne (FRA) 71-72-71-68
283 - Emiliano Grillo (ARG) 74-70-69-70, David Drysdale 74-72-67-70, Steve Webster 74-72-69-68, Chris Paisley 73-67-75-68
284 - Francesco Molinari (ITA) 73-71-70-70, Richie Ramsay 74-69-69-72, Joost Luiten (NED) 70-69-72-73, Paul Waring 71-72-71-70, Simon Wakefield 68-72-71-73, Oliver Fisher 73-69-70-72, Gary Lockerbie 69-76-69-70, Andrew Marshall 73-71-72-68, Andreas Harto (DEN) 71-67-73-73
Selected
285 - Richard Green (AUS) 75-71-68-71
286 - Garth Mulroy (RSA) 73-67-74-72
289 - Keith Horne (RSA) 77-69-72-71
290 - Justin Walters (RSA) 71-71-79-69
291 - Scott Arnold (AUS) 71-71-70-79
293 - Daniel Popovic (AUS) 72-72-71-78