He quickly grew out to become a prolific scorer from midfield, prompting a move to Vitesse in 1999 before being signed by Dutch giants Ajax in 2002, where he appeared in the UEFA Champions League group stage together with players such as Zlatan Ibrahimović, Rafael van der Vaart and Wesley Sneijder.
In the same season, at age 16, Sikora made his professional debut when he came on as a substitute for Gijs Steinmann in the 69th minute of the 2–1 away defeat to RKC Waalwijk on 15 February 1995.
As in the previous season, Go Ahead Eagles struggled, and with 22 points in 34 games the team finished in last place of the table thereby suffered relegation to the second-tier Eerste Divisie.
After another, for him personally, very strong 1998–99 season, in which he scored 13 goals in 29 league appearances games – but again failed to make promotion play-offs with the team, interest among Eredivisie clubs arose for Sikora, who was mostly utilised as a striker.
His biggest achievement during the season was reaching the round of 16 of the 1998–99 KNVB Cup, where the team was subsequently knocked out to PEC Zwolle after a 2–5 loss.
Sikora, who was mostly used in defensive midfield, made a total of 27 league appearances and five goals and was thus instrumental in Ajax securing their 29th title in club history.
Despite achieving successes with Ajax, the club's management decided that Sikora was to be loaned out in order for him to gain more regular playing time.
Both club and player struggled for form during the 2005–06 season, as he only made 13 league appearances, in which he remained goalless, as NAC finished 16th and only managed to avoid relegation following play-offs.
Plagued by injuries, Sikora struggled to make the starting lineup following his permanent move to NAC Breda and only made nine Eredivisie appearances during the 2006–07 season, in which he scored one goal.
In the following season, Sikora made 21 league appearances and scored two goals, but was still unable to match his performances of earlier years and decided to leave NAC as his contract expired.
[8] Soon after leaving NAC, Sikora moved to the United States, where he signed a lucrative contract with Major League Soccer (MLS) side FC Dallas with a base salary of US$144,000 and a guaranteed severance pay of just over US$163,000.
[11] He made a return after one month, but failed to establish himself as first-team regular and towards the end of the season he appeared during three league games, in which he was unable to make the final whistle and had to be substituted early on.
However, in the final game of the season, a 2–2 away draw against LA Galaxy, Sikora scored an important goal after an assist provided by Mexico international Duilio Davino.