[citation needed] On 30 June 2017, Tsimikas made another loan move, this time to Dutch club Willem II on a season-long contract.

In the quarter-finals of the KNVB Cup, his deflected free kick took the tie into extra time, and Willem II went on to beat Roda on penalties.

[citation needed] On 14 August 2021, he made his first league start in the 3–0 away win against Norwich City on the opening weekend of the season.

[19] On 13 April 2022, at the quarter-finals of the 2021–22 UEFA Champions League against Benfica, Tsimikas provided two assists in a 3–3 draw, helping Liverpool to secure their place in the semi-finals.

[24][25] He played in the 2024 EFL Cup final and assisted the winner for Virgil van Dijk from a corner kick.

[26] Tsimikas received his first call-up to the Greece senior team for the 2018–19 UEFA Nations League matches against Hungary and Finland in October 2018.

[27] He made his international debut in the starting eleven for the match at home to Hungary on 12 October, and crossed for Kostas Mitroglou to score the only goal of the game.