Siem had a pair of eagles to go with five birdies and a second-hole bogey in taking a three-stroke lead.
Winds drove up scores for the early players and only four players with morning tee times broke par. But Siem played in the afternoon and took advantage of better playing conditions.
After the bogey at the No. 2, Siem holed his approach shot at the par-4, 433-yard third hole for eagle and followed that with birdies at No. 4 and 5. He opened his second nine with birdies at 10 and 11 that tied him with Velasco for first.
He closed with an eagle at the par-5, 511-yard 17th hole and a birdie at 18 for the three-shot lead.