Slovakia close in on qualification
Goals in either half from forwards Stanislav Sestak and Filip Holosko put the Slovakians five points clear at the top of Group Three and sent Northern Ireland into third place, behind Slovenia on goal difference having played a game more.
Slovakia now need only to draw with Slovenia on Oct. 10 to ensure they qualify for next year's finals in South Africa.
Bochum's Sestak opened the scoring against the run of play after 15 minutes, tapping in from close range after great work from Manchester City winger Vladimir Weiss, son of the national team's coach of the same name.
Substitute Holosko settled the match, drilling home on 66 minutes to give Slovakia a second group win over their opponents.
Northern Ireland, unbeaten in their previous five qualifiers before Wednesday, wrap up their campaign with a trip to Czech Republic - two points behind in fourth with a game in hand - on October 14.