Siew won his first gold medal at the 1983 Southeast Asian Games in the 470 Men's Dinghy race. In the 1985 Southeast Asian Games, he competed in the Fireball class as a crew to Tan Tee Suan and won a silver medal. Siew went back to the 470 class and went on to win 5 consecutive gold medals at the Southeast Asian Games from 1987 to 1997, with different sailing crew.