Ajax topped the table for the first time this season with 54 points from 26 matches, one ahead of PSV, who slumped to a 2-1 defeat at Heerenveen on Saturday. Feyenoord also have 53 points following their 1-0 win at Roda JC Kerkrade.
Ajax, who drew their last two home matches, went ahead in the 19th minute when Lasse Schone set up Sigthorsson to score with a tight-angled shot.
Seven minutes later Zwolle keeper Diederik Boer blocked a header from Sigthorsson but was caught out on the rebound from de Jong. Derk Boerrigter added a third 10 minutes before time.
"Now we are in control but we still have to live match by match," Ajax manager Frank de Boer told reporters.
"The most important things is the way we are playing and that gives us so much confidence.
"Feyenoord are very close, as are PSV. Winning the last eight matches will earn the title."
Feyenoord tied up three points when Ruben Schaken's perfect cross was headed home by Graziano Pelle in the 30th minute.
Vitesse remain fourth on 51 points after Wilfried Bony's 63rd minute goal settled a 1-0 win at Twente Enschede, who slipped to sixth with 44 points.