Toornstra treble helps hammer Groningen
Toornstra opened the scoring in the 21st minute following a defence-splitting pass from Aleksander Radosavljevic and doubled the lead eight minutes after the break from close range.
Jonas Ivens pulled a goal back for Groningen with a fine header just past the hour but minutes later Lex Immers scored after a scramble to restore ADO's two-goal lead.
Toornstra then got his third goal before a long drive from Ramon Leeuwin forced a blunder from Groningen goalkeeper Luciano da Silva to wrap up the victory for the hosts.
The second leg is on Sunday at Groningen where the Den Haag side can claim a European place for the first time since 1987.