Siem countered three bogeys with seven birdies in the second round to get to 12-under 132 for the tournament.
Mikko Ilonen, despite a double-bogey 7 at the 15th-hole, turned in a 6-under 66 Friday and is tied with David Horsey for second at 9-under 135. Horsey also had a double bogey Friday, and ended the day with a 67.
Pablo Larrazabal piled up 10 birdies in shooting 8-under 64 Friday and jumped up to fourth at 8-under for the tournament.
Siem opened his second round with back-to-back birdies at Nos. 10 and 11. He added two more at 15 and 17 before stumbling to a bogey at the par-4 18th. He squeezed birdies at the fifth and sixth holes between bogeys at Nos. 4 and 7 and closed the day with a birdie at No. 9.