His first year as a senior was abandoned due to the COVID-19 pandemic, but earned him trials at professional clubs Rot-Weiss Essen, SC Freiburg and FC Schalke 04 before he opted for Fortuna Düsseldorf.
[2] Having scored three goals in seven games for TSG Sprockhövel while missing only three minutes, Niemiec joined Düsseldorf in May 2021, being assigned to the reserve team in the fourth-tier Regionalliga.
The game went to a penalty shootout, in which he was the only player to miss,[5] his attempt being saved by FCN goalkeeper Peter Vindahl Jensen.
[5][7] He played 12 games in his first league season, scoring once on 24 February to conclude a 3–1 win over Eintracht Braunschweig at the Merkur Spiel-Arena in his third match.
Bundesliga and contested the promotion/relegation playoff against VfL Bochum, surrendering a 3–0 away win from the first leg as the game went to penalties; Niemiec came on as a substitute and scored his attempt while his team lost in sudden death.