Sergey Demyashkevich took up wrestling in 1977 and made the Soviet national team in 1988. After winning the 1988 World Cup and finishing fifth at the 1989 World Championships in Greco-Roman light-heavyweight, Demyashkevich switched to heavyweight and wrestled in that class until the end of his sporting career. As a heavyweight, Demyashkevich won Olympic bronze in 1992, was World Champion in 1990, took bronze at the 1991 World Championships and was European Champion in 1991 and 1993. Domestically, Demyashkevich won the Soviet title in 1991 and the CIS title in 1992. Demyashkevich graduated from the Belarusian State University with a law degree and, after his retirement from sports, worked as a lecturer with his alma mater. He also became the vice-president and general secretary of the Belorussian Wrestling Federation.