Hendrickx made his senior international debut for Belgium in 2012, at the Champions Trophy. He was a reserve player at the 2016 Summer Olympics, where Belgium won a silver medal. In November 2018, he was named in the squad for the 2018 World Cup in Bhubaneswar, India. At the tournament, he finished as top scorer alongside Blake Govers of Australia with 7 goals. At the 2019 EuroHockey Championship, he also was the top goalscorer together with three other players with five goals. On 25 May 2021, he was selected in the squad for the 2021 EuroHockey Championship. Alexander Hendrickx is sponsored by Y1 Hockey. He uses the LB X and LTD X, this is a low bow stick which is well suited to his drag flicking technique. He was top goal scorer at Tokyo 2020 with 14 goals using the LTD X.