David Mendes da Silva came back to haunt Sparta Rotterdam, scoring the only goal of the game to keep AZ Alkmaar's push for fourth place very much alive.
The former Sparta player's early goal saw AZ get off to the perfect start and the hosts could not find the equaliser.
AZ are now three points behind Feyenoord, who have a game in hand in the race for the last automatic qualifying place for Europe, while Sparta slip to just a point off the relegation play-offs following their third successive defeat.
The visitors had not lost since January and were looking for their fourth win on the trot.
And they got off to the perfect start in the fourth minute when Nick Viergever headed out Maarten Martens' cross but only as far as Moussa Dembele who put the ball back in the box.
Aleksander Seliga punched out Mounir El Hamdaoui's header but, with the Sparta goalkeeper on the ground, Mendes da Silva fired his side into the lead.
Sparta should have levelled 10 minutes later when Kevin Strootman spotted Joey Didulica - who had replaced the injured Sergio Romero moments earlier - off his line and put in a tantalising cross but AZ winger Simon Poulsen rushed in to fire the ball inches wide of the right upright, only narrowly avoiding putting the ball into his own net.
Didulica did better in the 34th minute when he did well to keep out Charles Dissels' powerful free-kick which was almost deflected past him.
AZ almost stunned their hosts with a second moments after the interval as, much to everyone's surprise, El Hamdaoui turned and shot from distance but fired narrowly over.
Dissels once again came close to equalising as Kevin Strootman rolled a free-kick towards the Sparta striker, whose thunderbolt of a 35-yard strike flew inches wide of the right upright.
And in the end Sparta were made to rue their missed opportunities.