Demyanenko qualified for the men's 100 m breaststroke at the 2008 Summer Olympics in Beijing, by clearing a FINA B-standard time of 1:03.59 from the Belarus Swimming Championships in Minsk. He challenged seven other swimmers on the third heat, including three-time Olympians Malick Fall of Senegal and Alwin de Prins of Luxembourg. Demyanenko rounded out the field in last place by two thirds of a second (0.67) behind Barbados' Andrei Cross with a time of 1:05.14. Demyanenko failed to advance into the semifinals, as he placed fifty-sixth overall in the prelims.