With Dutch international legend and current Heerenveen coach Marco van Basten set to take over the reins at AZ in the close-season, the tie in pursuit of the final UEFA Europa League place was always going to have plenty of talking points.
However, the form book was turned upside down as AZ, without a win in their previous eight games in all competitions, struck three times in the opening 39 minutes to stun their visitors.
Goals from Paraguayan midfielder Celso Ortiz, a Numenja Gudelj penalty and Steven Berghuis, set up an intriguing second leg at the Abe Lenstra Stadion on Saturday.
In Wednesday's earlier game, a late strike from Lorenzo Burnet gave Groningen a slender 1-0 first-leg advantage over Vitesse.
The spot-kick was awarded for a foul on Ecuador midfielder Renato Ibarra, but Christian Atsu's kick was saved by Groningen goalkeeper Marco Bizot.
The game was settled in the 88th minute when Tjaronn Chery's cross was not cleared and Burnet pounced to score.
