In the 2001–02 season, Diamantopoulos led the European-wide 2nd-tier level FIBA Saporta Cup in scoring, averaging 22.6 points per game.

In the 2006–07 season, Diamantopoulos won the FIBA EuroCup Challenge championship, while playing with the Russian Super League club CSK VSS Samara.

Diamantopoulos was a member of both the Under-16 and Under-18 junior Greek national basketball teams.

With Greece's national junior teams, he also played at the 1999 FIBA Under-19 World Cup.

He was also a member of the senior men's Greek national basketball team.