The win moves Ajax to 78 points from 32 games and ahead of defending champions PSV Eindhoven on goal-difference with two matches left to play.
Milik struck first from the penalty-spot on 57 minutes after a handball in the Heerenveen box, before adding a second shortly afterwards to take his league tally for the season to 21.
Third-placed Feyenoord were held to a 2-2 draw by Heracles as they were twice forced to come from behind, perhaps their thoughts already on Sunday’s Dutch Cup final against Utrecht.
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PEC Zwolle moved into the UEFA Europa League playoff positions with a 2-0 home win over NEC Nijmegen, while Twente Enschede added to the relegation woes of Excelsior Rotterdam with a victory by the same scoreline.
Excelsior are one point above the relegation playoff positions, but have played a game more than Willem II below them, who are in action against bottom side Cambuur on Thursday.
(Reporting by Nick Said in Cape Town, editing by Pritha Sarkar)
