Siem started the tournament on the back nine and his round included six birdies and a 20-foot eagle putt at the par-5 ninth hole.
"I cannot remember the last time that I shot 64 to be honest with you," said Siem, a one-time winner on the European Tour. "It must be at least three years ago, so I am pretty happy with that. It's definitely my best round for a few years."
Chan Yin-shin of Taiwan shot 7-under 65 and is alone in second place. Robert-Jan Derksen, Andrew Dodt, Rahil Gangjee, Chih-Bing Lam, Barry Lane, John Parry and Thaworn Wiratchant all shot 5-under 67 and share third.
Jeev Milkha Singh, the highest-ranked player in the field at No. 59, shot even-par 72 and is tied for 57th heading into the second round.