Early years In his teenage years Nikolaidis played for
Ethnikos Alexandroupoli, the local team of his hometown. The scouts of several teams had seen his progress from these years, earning him a move to
Athens. He made his professional debut at the age of 20 at
Apollon Athens. With Apollon, he reached the
Greek Cup final on 15 May 1996, where they were defeated with the record score of 7–1 from
AEK Athens. His performances sparked a bidding war between
Panathinaikos and
Olympiacos, as well as AEK Athens, which was a supporter. Nikolaidis refused to join Olympiacos and forced his club president to sell him to AEK. Thus, on 20 June 1996 he was transferred to AEK for 330 million
drachmas and
Pantelis Konstantinidis as an exchange.
AEK Athens At AEK, Nikolaidis linked up with some of the Greek international players of his generation, including
Vasilios Tsiartas,
Theodoros Zagorakis,
Michalis Kapsis,
Grigoris Georgatos,
Traianos Dellas,
Vasilios Lakis. With his new side, Nikolaidis excelled, scoring on his debut against
Ionikos. During his time with AEK, Demis managed to win three Greek Cups and the 1996
Greek Super Cup, while he was the top scorer in the
1998–99 Greek league and second scorer in the
2000–01 UEFA Cup (1 goal behind
Dimitar Berbatov). He is the top scorer of Greek clubs in European competitions having scored 26 goals in 51 games. He has scored five goals in a match twice and is the only Greek player to have scored 4 goals in a European match. Having scored a total of 21 goals in the
UEFA Cup he is one of the competition's all-time topscorers. At AEK he scored 190 goals in 266 games (including European, cup and domestic league games) making him the fourth highest goalscorer for the club. On 24 March 2002, the
International Committee for Fair Play awarded him with the Fair Play Award, an honorary diploma for his conduct in the
Greek Cup Final on 8 May 2000, between the teams of AEK Athens and Ionikos, when Nikolaidis informed the referee he had used his hand to score a goal that had been allowed as valid. In January 2003, Nikolaidis filed a lawsuit against the club boss, Makis Psomiadis, who had come by his house at 3 a.m. to check if he was staying overnight and also for blackmail, insult and threatening his life and physical integrity. During the incident he also stated that apart from Psomiadis, his bodyguards were also present. That incident resulted in his decision to leave the club at the end of the season. He left the club in a mutual consent, even though he was already paid for the rest of his contract.
Atlético Madrid On 11 July 2003,
Atlético Madrid, after beating several other clubs evantually landed the striker's coveted signature. Seeing that his number 11 was taken, he wore the number 21 as a tribute to AEK and the
Original 21 fan club. 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. However, a series of serious injuries kept him out of the first team for nearly the rest of the season and severely affecting his chances of getting picked for
Euro 2004. Although Atletico wanted to keep him to the roster of the team, Nikolaidis decided to retire from professional football. ==International career==