Brazilian Jonathan Reis hit a hat-trick with Ibrahim Afellay, Ola Toivonen, Jeremain Lens (2), Balazs Dzsudzsak (2) and Orlando Engelaar joining in the rout.
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Feyenoord, traditionally one of the 'big three' clubs in the country and European Cup winners in 1970, were reduced to 10 men on 34 minutes when midfielder Kelvin Leerdam was sent off.
"This really hurts, suffering a defeat like this and it was very emotional in the dressing room afterwards," Feyenoord coach Mario Been told reporters.
"It is hard to realise what happened," added left back Tim de Cler. "After the first goal and the red card everyone just played their own match and we totally collapsed."
Ajax Amsterdam slipped to third with 21 points after a 2-2 draw at Excelsior Rotterdam, who took a 2-0 lead after Ajax's Vurnon Anita was sent off.
