Ajax have 60 points from 28 matches, three more than PSV Eindhoven, who drew 2-2 at Roda JC Kerkrade, and Vitesse Arnhem, who won 2-1 at home to PEC Zwolle.
Feyenoord slipped to fourth with 56 points following their 2-0 defeat at Heerenveen on Saturday.
Fischer opened the scoring for hosts Ajax after 21 minutes when the 18-year-old picked up a pass from Siem de Jong and broke through the centre of the Nijmegen defence before an own goal by Rens van Eijden doubled the lead.
Fischer added his third 13 minutes from time with a superb lob from outside the area, while Ryan Babel scored Ajax's fourth after Nick van der Velden pulled one back for the visitors.
"Viktor is an excellent player who has excellent qualities if you see his second goal," Ajax manager Frank de Boer told reporters.
"This was a good weekend and we are in control in the league but we still approach it match by match, so first Heracles Almelo before we travel to PSV."
PSV seemed on track to stay in touch with the leaders when Ola Toivonen replied with a first-half double after Frank Demouge had headed Roda in front in the 13th minute.
Sanharib Malki, however, salvaged a point for lowly Roda three minutes from time after finishing a pass from Mikolaj Lebedynski.
First-half goals by Wilfried Bony and Mike Havenaar proved sufficient for Vitesse in their 2-1 win over Zwolle, who pulled one back through Arsenio Valpoort in the 78th minute.
In the day's other action, RKC Waalwijk cruised to a 4-0 win over visitors ADO Den Haag and AZ Alkmaar, who were reduced to 10 men in the 31st minute after the dismissal of Thomas Lam, beat hosts Heracles Almelo 2-1.