Siem, whose last victory came eight years ago at the Dunhill Championship, didn't record a bogey until the final hole, finishing at at 8-under 276.
That was good for one-stroke victory over Francesco Molinari, who carded a 7-under 64.
The German essentially sealed the victory with a 4-foot birdie putt at the 15th hole.
France's Raphael Jacquelin stumbled to three bogeys over his last 10 holes, ending with a 2-under 69 for third place at minus-6.
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