In an absorbing encounter, the Steelers came from 3-1 down at the first break to fire four second-period goals to find themselves 5-4 ahead at the second break.
Cardiff equalised early in the third period, but both goalies dominated which meant neither side could find a way through again.
After one goalless period of overtime another seemed likely until with 5 minutes and 52 seconds left on the clock, Nelson broke down the middle to fire high past Ben Bowns in the Cardiff net much to the delight of the thousands of Steelers fans in the crowd.
As starts to a game go, it had been pretty explosive.
An early flashpoint arrived in the shape of John Armstrong who, for the second week running, found himself involved in a collision with a netminder.
Just 36 seconds had elapsed when he crashed into Ben Bowns in the Devils goal under pressure from a Cardiff defender, bringing back memories of last week’s collision in the quarter-final with Nottingham Panthers’ goalie Miika Wiikman.
On that occasion, Wiikman was stretchered from the ice after seemingly being knocked out cold, while Steelers’ Armstrong was handed a game misconduct.
On this occasion, he was just handed two minutes for interference, but unlike last week when the Steelers killed off the subsequent five-minute penalty - even scoring short-handed through captain Jonathan Phillips - on this occasion, their penalty kill couldn’t get the job done, Guillaume Doucet firing past Ervins Mustukovs through a screen for the lead at 2.20.
But Armstrong quick to fire back for his team, picking up a loose puck just inside the halfway line before bearing down on Bowns and firing a rocket over the Rotherham-born netminder’s left shoulder.
Chances came and went at both ends, the clearest for the Steelers coming to the stick of Colton Fretter, who was somehow denied by Bowns from out in front.
At the other end, David Brine was denied by an alert Mustukovs putting the subsequent rebound wide of the right-hand post.
Cardiff began to get on top slightly towards the end of the period and they went ahead thanks to a quick break by Joey Haddad, who fired past Mustukovs before the Steelers goalie could get set.
Worse was to come in the final minute of the period, however, when - with Guillaume Desbiens in the box on an interference call - Martin made it 3-1 to the Devils.
If onlookers thought the first period was, at times, frantic, nothing prepared them for what came in the next 20 minutes.
The Steelers came out flying, no doubt on the back of some choice words from head coach Paul Thompson during the interval.
An early powerplay from the Steelers brought nothing - despite good chances for Desbiens, O’Connor and Armstrong. But, soon after, they got their reward, a seemingly speculative shot from Geoff Walker eluding Bowns to make it 4-2 at 24.24.
Just over a minute later, Colton Fretter danced his way towards Bowns before poking the puck through the goalie’s legs.
It got even better in the 29th minute when the Steelers went ahead for the first time in the match, Levi Nelson tipping in from Ben O’Connor’s shot.
The Steelers’ joy was short-lived, however, when Layne Ulmer scored past Mustukovs at the other end at 32.20 after Joey Haddad broke free down the left and cut inside.
But the Steelers were to regain the advantage before the end of the period, Walker reacting quickest to poke slot the puck past Bowns at 37.23.
It wasn’t long before the Devils were level - again - however, Andrew Hotham tipping in past Mustukovs on a 4-on-3 powerplay at 40.54.
Jonathan Phillips and Fretter came closest for the Steelers while at the other end, Mustukovs had to be alert to deny Haddad, while Patrick Asselin hit the eight-hand post from just a couple of feet out, at a tight angle.
Both sides came out intent on finishing the job as quickly as possible, with plenty of chances in at both ends, but both goalies were unflappable, with Bowns probably been kept the busiest of the two.
The Yorkshireman had to be alert to deny John Armstrong right in front, but the video review was needed to judge whether the Devils netminder had indeed denied Markus Nilsson. He had and with Fretter denied again shortly after, another 20 minutes of overtime was required.
And it looked as if another 20 minutes would be required until Nelson came up with his moment of magic.