Amidst a long series of "derbies of the eternal enemies", opposing Panathinaikos and Olympiacos Piraeus, Diamantidis thrived, as he faced Olympiacos every year for twelve consecutive seasons, including in eleven Greek League Finals, six Greek Cup Finals, and a EuroLeague semifinal. He won nine Greek League and ten Greek Cup titles, alongside a record six Greek League Finals MVP and a record six Greek League MVP awards, as well as two Greek Cup MVP awards. Diamantidis' personal friendship and rivalry with Olympiacos star Vassilis Spanoulis, was at the center of attention throughout six Greek League Finals and three Greek Cup Finals encounters. Diamantidis' self-effacing psyche helped relieve tension between the two powerhouses, when he retired in 2016.