Sideris began his youth career at Atromitos Piraeus playing in the centre of midfield but his coaches soon became aware of his goalscoring capabilities.
When he was promoted to the senior team in 1958 he produced an outstanding debut season in which Sideris became the fulcrum of Atromitos’ attack.
The youngsters direct, no-holding-back attacking style plundered 28 goals in 30 league appearances Sideris transferred to Olympiacos in the summer of 1959.
Despite having his heart set on a move to Panathinaikos, a friend of his, Savvas Theodoridis, had ushered him towards Olympiacos, fresh off winning their sixth consecutive league title.
In 1970, he transferred to Belgium's Royal Antwerp[3] after having finished second to Bulgaria's Petar Zhekov of CSKA Sofia (36), for the European Golden Boot competition in 1969, scoring 35 goals in that season.