Siem moved to 6-under-par 136 at the midway point of the event in a bid to win his second title on the European Tour. He captured the Dunhill Championship in 2004.
First-round leader Christian Nilsson stumbled to a 75, leaving him in a tie for eighth at 140.
Rain and high winds forced an 80-minute delay in the action.
"If you shoot a few under on the golf course, it doesn't matter what the conditions are, it's a good score," Siem said.
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