Frank de Boer's men boosted their hopes of a fourth successive title with a slender victory at De Adelaarshorst, midfielder Laase Schone's second-half strike proving the difference between the two sides.
The Denmark international found the bottom corner with a vicious long-range effort after his initial free-kick had been blocked by the wall.
And the capital club now have some breathing room at the summit of the division after Vitesse, who had previously trailed Ajax on just goal difference, failed to overcome bottom club NEC.
Peter Bosz's side appeared set for a routine victory as defender Dan Mori headed home Christian Atsu's corner on the half-hour mark.
However, NEC refused to lie down and earned a point in the 71st minute through an emphatic finish from Michael Higdon, the striker scoring his seventh league goal of the season.
Heerenveen's chances of securing automatic qualification for the UEFA Europa League were dented with a 3-1 defeat at Cambuur.
Elvis Manu gave Cambuur the lead inside four minutes, before Bartholomew Ogbeche doubled the advantage a minute before the interval.
Jody Lukoki added a third midway through the second half to seal the points for Dwight Lodeweges' side, despite Alfred Finnbogason's late consolation.
Elsewhere, Groningen's clash with Twente was postponed due to a frozen pitch.
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