The German had eight birdies and three bogeys, including one at the 18th, to finish at 9-under 135.
I was pretty calm, which is unusual for me, Siem said. I said to my caddie that I felt like Retief Goosen ' I dont have any emotions.
Jean-Francois Lucquin of France (69) and Graeme Storm (66) of England were a stroke back. Local favorite Carlos del Moral (67), David Drysdale of Scotland (67) and Swedens Michael Jonzon (68) were all at 7 under.
Scottish rookie Chris Doak (73), the first-round leader, is a further shot back in a group that includes Miguel Angel Jimenez of Spain (65) and Colin Montgomerie (72).
Montgomerie, who is playing in his 500th European Tour event, had an inconsistent day with five birdies and five bogeys and had words with a photographer at the Real Club de Golf de Sevilla.
I played very badly, and 72 is what you get when you play that way, Montgomerie said.