In a sterling career with Ethnikos Alexandroupolis, Apollon Smyrnis, AEK Athens and Atlético Madrid, Nikolaidis earned his reputation as a "born goalscorer", scoring prolifically for club and country.

[4] After quarrelling with owner Makis Psomiadis, later indicted for forgery, and allegedly being assaulted by his bodyguards, Nikolaidis decided to leave AEK after the 2002–03 season.

Nikolaidis' first months at the Vicente Calderón Stadium were a tremendous success, he scored six goals and formed a fearsome attacking pair with teenage sensation Fernando Torres.

New coach Otto Rehhagel focused his offense around the quick striker and was rewarded with direct qualification to the Greek team to represent Greece at Euro 2004.

[6] With AEK struggling in the wake of Psomiadis' corruption and the prospect of relegation to the fourth division looming, Nikolaidis retired at the relatively young age of 31.

He and technical director Ilija Ivić, a former teammate, made several clever signings and fought ferociously for the league title, finishing third when many had expected a mid-table performance.

In his second year as president, after signing two promising Greece Under-21 players, as well as adding former star Vasilis Lakis, Ukrainian international striker Oleh Venhlinskyi and one time Inter centre back Bruno Cirillo, AEK achieved UEFA Champions League qualification.

[8] On 2 November 2008, Demis decided that he would quit as chairman of AEK after continuously poor results, stating that he had not achieved his goal in the 5 years plan.

Nikolaidis said one of his reasons for quitting was that the team didn't sell 30,000 season tickets; he stated "If I had managed to convince the AEK supporters to come to the stadium then I would not be leaving.

During Nikolaidis's presidency, AEK had their first wins in Champions' League and the club made about 12,000,000 euros from selling players to other teams (like Kostas Katsouranis, Sokratis Papastathopoulos, Dániel Tőzsér, Sotirios Kyrgiakos etc.).