William Kyriakos Jr. (born 3 September 1972) is a Brazilian former professional tennis player. Kyriakos, who comes from São Paulo, represented Brazil at the 1991 Pan American Games in Havana. He won a silver medal in the mixed doubles competition, partnering Cláudia Chabalgoity, as well as a gold medal in the team event. The team of Kyriakos, Nelson Aerts and Marcelo Saliola defeated Puerto Rico in the final.