Locked in a six-team battle for the crown, the Amsterdam club took over top spot from AZ, which was later held 2-2 by Vitesse Arnhem.
Frank de Boer's men opened the scoring in the first half through Theo Janssen and then cut loose in the second, with Lorenzo Ebecilio making it 2-0 and Christian Eriksen claiming the third despite a possible deflection off Ebecilio.
Vurnon Anita, Siem de Jong and Kolbeinn Sigthorsson - on as a substitute having been out injured since early October - completed the rout.
That put the pressure on AZ to respond and it was frustrated at the Gelredome.
United States forward Jozy Altidore headed it in front but Guram Kashia (35) and Marco van Ginkel (46) gave Vitesse Arnhem the lead.
Simon Poulsen was credited with the equaliser on the hour but Brett Holman almost immediately collected a second yellow card and AZ's remaining 10 men was unable to find a winner.
De Graafschap remains at the foot of the table after a 2-0 defeat at NEC Nijmegen.
Going into the match two points behind Excelsior, Richard Roelofsen's men knew a three-point haul would lift them off the bottom of the pile.
But goals either side of the break from Lasse Schone and Genero Zeefuik ensured that NEC coasted to a comfortable success, aided by Lion Kaak's dismissal just prior to half-time.
RKC Waalwijk boosted its hopes of clinching a UEFA Europa League play-off place with a narrow victory over ADO Den Haag.
Krisztian Nemeth netted the game's only goal midway through the first half to lift RKC back level on points with eighth-placed NEC.