Feyenoord then squandered a chance to hit straight back. Ajax right back Ricardo van Rhijn conceded a penalty and was sent off just a minute after De Jong's goal for a foul on Jean-Paul Boetius, but Kenneth Vermeer saved Lex Immer's spot kick and the Feyenoord midfielder blasted the rebound over the bar.
While Ajax won, its main title rivals dropped points. PSV slumped to a 3-1 home defeat against PEC Zwolle on Friday, while league leader FC Twente was held to a goalless draw by RKC Waalwijk on Saturday and Vitesse Arnhem lost to AZ Alkmaar.
Niklas Moisander set up the opening Ajax goal in the seventh minute with a clever pass to Fischer who coolly rounded Feyenoord keeper Erwin Mulder to score. Fischer doubled the lead in the 40th thanks to sloppy defending by Joris Mathijsen. The Feyenoord and Netherlands defender hit the ball against Fischer as he attempted a clearance and could only watch as the young Dane notched his second.
In other matches Sunday, sixth-placed Utrecht beat Groningen 2-0 for its fifth road victory of the season and Willem II drew 1-1 with ADO The Hague.