Heerenveen, Netherlands – AZ Alkmaar suffered just its second 1, at the Abe Lenstra Stadion.
Bas Dost opened the scoring for Heerenveen in the 14th minute but it was quickly canceled out by Jozy Altidore six minutes later.
Heerenveen then went on a four-goal tear, as Rajiv van La Parra, Filip Djuricic, Ramon Zomer, and Luciano Narsingh each popped up with a tally to ensure a crippling defeat for Eredivisie-leading Alkmaar.
Fortunately for Alkmaar, second-place PSV also suffered a defeat, losing to Feyernoord, 2-0, on Sunday behind goals from Sekou Cisse and Ruben Schaken.
AZ Alkmaar remains in pole position with 34 points, followed closely by PSV on 31 points.
Third-place Twente made up some ground on the top-two clubs with a resounding 6-2 victory over Utrecht on Sunday. Ola John and Willem Janssen both scored twice en route to the convincing win.
And finally, Vitesse claimed a 4-0 win over RKC Waalwijk to move into a Europa League playoff position. RKC Waalwijk is winless in its last eight league contests.