Northern Ireland have taken a 1-0 lead into half time away in Finland thanks to a header from Craig Cathcart.
Stuart Dallas set the tone for Northern Ireland by shooting in the opening two minutes but his near-post effort flew over the bar.
Niall McGinn then attempted to thread a pass through to Kyle Lafferty, though the ball was just an inch too far ahead of the Norwich City striker.
A mistake from Finland keeper Lukas Hradecky provided Leeds United winger Dallas with an opportunity to open the scoring but by taking a touch before shooting, Dallas allowed Hradecky the chance to recompose himself and save the effort.
Both Chris Brunt and Chris Baird saw goalbound efforts blocked by their own teammates, while Tim Sparv dragged a half-volley wide for the hosts.
Brunt's 31st-minute corner was then cleared as far as McGinn, who made space down the right before sending in a cross which Watford defender Cathcart nodded in to give Michael O'Neill's side the lead.
Finland responded immediately as Joel Pohjanpalo tested Michael McGovern in the Northern Ireland goal, whose fingertip save kept his team in front at the break.
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