[4] Ahead of the 2021–22 season, Hartjes and fellow academy player Antoni Milambo were promoted to first team by head coach Arne Slot.
[5] He made his professional debut for Feyenoord in a 3–0 UEFA Europa Conference League win over FC Luzern on 12 August 2021, replacing Orkun Kökçü in the 58th minute.
[7] He made his debut for the club as a starter in defensive midfield in a 3–0 away win in the league over FC Den Bosch after a hat-trick from former Feyenoord teammate Dylan Vente.
[8][9] On 18 October, Hartjes scored his first competitive goal at senior level, providing the 2–2 equaliser in a KNVB Cup match against Heracles Almelo.
[10] Despite keeping Roda's hopes of advancement alive, Hartjes would later miss the decisive penalty in the shootout, knocking the club out of the cup competition.