With the top two slipping up this weekend, when leaders Twente Enschede were held to a goalless draw by RKC Waalwijk and second-placed PSV Eindhoven lost 3-1 to PEC Zwolle, Ajax were able to capitalise despite ending their match with 10 men.
The victory literally brought the house down as there were dramatic scenes in the dying minutes when substitutes and coaches ran on to the pitch to avoid being covered by a huge amount of snow that suddenly crashed through the stadium's roof.
The victory put Ajax level with PSV Eindhoven on 40 points from 19 games and one point behind Twente.
Ajax opened the scoring in the seventh minute when Christian Eriksen set up Fischer, who slipped past his marker and rounded keeper Erwin Mulder.
More defensive sloppiness by Feyenoord handed Ajax and Fischer their second five minutes before the interval when the 18-year-old intercepted a pass from Joris Mathijsen and finished with a tight-angled shot.
Fischer's Danish compatriot Eriksen added a third goal after 62 minutes when a 20-metre free-kick surprised Mulder at the near post.
"Our efficiency was the basis for this win, but we were much too sloppy in possession of the ball despite all the space we got," Ajax manager Frank de Boer told reporters.
A minute after the third goal Feyenoord had the chance to get back into the game as Ricardo van Rhijn was sent off after a second booking but Lex Immers missed the resulting penalty.
"In matches like this... you can't afford to make the mistakes we made," Feyenoord manager Ronald Koeman told reporters.
"The 3-0 scoreline is a bit flattering as we had our moments and could not convert a penalty."
Feyenoord stay fourth on 37 points, two above Vitesse Arnhem who suffered a 4-1 defeat at AZ Alkmaar on Saturday. Utrecht are sixth with 33 points following a 2-0 win at Groningen with Alexander Gerndt and Mike van der Hoorn on target.
