KNVB Beker wrap: Groningen edge ADO Den Haag
In the only third-round match of the premier Dutch knockout cup competition involving two Eredivisie clubs, De Leeuw's strike proved the difference on Tuesday.
Despite sitting three spots below eighth-placed Den Haag, Groningen scraped into the Round of 16 thanks to De Leeuw hitting the back of the net in the 33rd minute.
In other results, ADO '20, a club from the third tier of Dutch football, smashed second-division club Eindhoven 6-1 on the back of a stunning start to the game.
After just six minutes, ADO '20 led 3-0 with Benny Kleine, Franky Zinhagel and Mark de Loor all scoring as Eindhoven fell apart.
Kleine added his second goal in the 11th minute and two minutes later it was 5-0 when Zinhagel also completed his brace.
Serhat Koc got Eindhoven on the scoresheet in the 35th minute but the visitors failed to score again, with ADO '20 wrapping up their huge win when Jeffrey Mols struck with 29 minutes to play.
In other KNVB Beker results, Go Ahead Eagles knocked off De Graafschap 3-0 and Cambuur won on penalties after their match with Telstar finished 1-1 through 120 minutes.
Dordrecht defeated Scheveningen 2-1 and Rijnsburgse Boys triumphed on penalties after their match against Emmen remained scoreless after extra time.