Feyenoord's title defence suffered another setback on Sunday as Kasper Dolberg came off the bench to score twice and inspire Ajax to a 4-1 win at De Kuip.
It represented a third league defeat of the campaign for the Rotterdammers, who did not lose a third last season until April and they now trail leaders PSV by eight points.
Substitute Dolberg highlighted a four-goal second half for Ajax, sparked by three assists from David Neres in a dominant man-of-the-match display from the Brazilian winger.
Klaas-Jan Huntelaar opened the scoring for Marcel Keizer's side in the 50th minute following a disjointed first half – his ninth Klassieker goal – but Feyenoord levelled just eight minutes later through Jens Toornstra, after Nicolai Jorgensen missed a penalty for the hosts minutes earlier.
De Kuip was bouncing but the home fans were soon silenced again as Dolberg - who replaced Huntelaar - restored Ajax's lead in the 72nd minute and added to it in stoppage-time, with Siem de Jong also finding the net to seal a dominant victory, their first league win at De Kuip since 2014.
Koning in de Kuip! October 22, 2017
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