The penalty was awarded following a VAR review, which showed that Willem II goalkeeper Thomas Didillon-Hödl had unfairly challenged Brian Brobbey. Klaassen confidently stepped up and converted from the spot.
This win marks Ajax's sixth league victory of the season. The Amsterdam side moves above FC Twente in the standings and still has two games in hand. Next up is the rescheduled clash against Feyenoord, with the Klassieker kicking off in Rotterdam at 18:00 on Wednesday.
Ajax - Willem II 1-0 (1-0)
06. Klaassen (pen.) 1-0
Ajax: Ramaj; Gaaei, Rugani (64. Šutalo), Kaplan, Baas, Wijndal (64. Taylor); Tahirović (77. Godts), Van den Boomen, Klaassen (84. Hato); Traoré (77. Henderson), Brobbey