1967 : 4th SEAP Games -- 6 golds (4 individual 2 team);
broke 4 games records

Tan Thuan Heng was a member of the Chinese Swimming Club where he developed his swimming prowess. At the tender age of 13, he was already representing Singapore in the Southeast Asia & Peninsular Games (SEAP Games) in 1961 and won the 400m freestyle event in a new record time. Thuan Heng went on to represent Singapore in the Tokyo Olympics in 1964. He also competed at the 1962 Empire Games and the 1966 Asian Games in which he won two silver medals in the freestyle events. His father, Tan Kay Hai, was an important driving force behind his achievements. Thuan Heng was to become one of the rare few to obtain a Rhode Scholarship in 1971.

He was also breaking a series of longstanding swimming records and set a national 100m freestyle record with a time of 56.3 seconds in 1969 which stood until 1977. In all, he won 11 individual gold medals at the SEAP Games. His best year was in 1967 when he had four individual gold medals in Bangkok . With this win, he became the first to receive the inaugural Best Sportsman of the Year award in 1967.