Siem is at 9-under-par 135 after 36 holes while Jean-Francois Lucquin and Graeme Storm were tied for second at 8-under. Carlos Del Moral, David Drysdale and Michael Jonzon are tied for fourth at 7-under.
Siem started Friday's round with a birdie at No. 1 -- the only hole he bogeyed Thursday -- and moved to 3-under on the round with birdies at Nos. 4 and 5. He gave up a stroke at the sixth hole but got that back with a birdie at 9.
He had four birdies in a six-hole span of the back nine -- a stretch in which he also had a bogey -- and finished with another bogey as he three-putted the 18th green for a 5-under 67.
"It's been a long time since I made eight birdies," said Siem. "Today was actually a bit better than yesterday, not only because of the score, but I had a few more chances."
Siem had a 68 and was tied for sixth after Thursday's first round.