A superb hat-trick from Denny Landzaat inspired the home side to victory to leave Sparta in trouble second from bottom of KPN Eredivise.
The midfielder netted his first after just four minutes and Mariano Bombarda doubled the lead three minutes later.
Landzaat grabbed his second from the penalty spot in the 16th minute and he completed his hat-trick and the game's scoring with the fourth on the stroke of half-time with another penalty.
Max Huiberts last-minute goal gave AZ Alkmaar a last-gasp 2-1 win over De Graafschap at the Alkmaarder Hout stadium.
Robin Nelisse had given the home side the lead after 27 minutes, but Hans van der Haar drew the sides level in the 37th minute.
The game looked set to end in a draw until Huiberts' late intervention and the win lifts AZ up to 10th spot in the table.
Click on fixture for full match summary
AZ Alkmaar 2-1 De Graafschap
Willem II 4-0 Sparta Rotterdam
Click here for updated league table
