Duncan played one year for the Clemson University. He reached the singles final of the National Collegiate Athletic Association, in which he defeated Mikael Pernfors. He was then elected to the best selection of All-American. Towards the end of 1984, he was a professional tennis player. The following year, he reached the final of an ATP tournament three times, but lost all three games. 1986 was much worse, his best results were quarterfinals in Bari and Saint -Vincent. Because of injuries, he was able to deny in 1987 only a few games. Since he had slipped in the world rankings, he played on the ATP Challenger Tour; there he was able to win the tournaments of Agadir and Casablanca for themselves. Between 1988 and 1990 he was still a further three times in a final on the ATP World Tour. Throughout his career, he never was able to achieve a victory. Its highest listing in the tennis world ranking he reached in 1985 with position 47 in the singles and 1987 position 182 in doubles.