It was to be the last action for the Plymouth Argyle keeper who, after receiving treatment in the first half following a collision with Rasmus Hojlund, was replaced by Pierce Charles at the break.
Ten minutes into the second half, the Sheffield Wednesday man made the first save of the game when he tipped Christian Norgaard's header round the post.
From the resulting corner, Ballard then Bradley cleared Danish efforts off the line.
With the pressure building on his side, Michael O'Neill brought Jamie Donley and George Saville off the bench but with 23 minutes to go Eriksen put Denmark ahead.
The midfielder, who will leave Manchester United this summer, was on hand to tap home after Isaksen's ball fell kindly for him after touches off both Trai Hume and Ballard.
Isaksen forced Pierce Charles into a diving save shortly after and substitute Mika Biereth had a headed goal ruled out for offside as the hosts stepped up their intensity.
With Bradley, who had always looked the most likely to create something for his side, replaced for the final 17 minutes, Isaac Price's speculative strike from distance that flew well over the crossbar was the closest Northern Ireland came to an unlikely equaliser.
The side are next in action on Tuesday night when they will host Iceland in Belfast in what is their final game before their World Cup qualifying campaign begins in September.