Top two seeds Tappara and KooKoo booked their passage to the semi-finals 24 hours earlier, but the third seeds, SaiPa, are under threat of missing out after going down 5-1 at home to Assat.
Assat had already secured one away win earlier in the series, and the belief was clearly with the side early on in Lappeenranta - there were six minutes on the clock when Feetu Knihti stole the puck in the attacking zone and laid it on a plate for Roope Talaja to finish with aplomb.
1-0 was how things stayed until the fourth minute of the second period when the same two players combined - Knihti took control from Petteri Nurmi at centre ice and played another perfect pass for Talaja to slot home.
The home side had hope four minutes later when Brian Kramer's shot from the blue line flew in to halve the deficit, but the lead was back to two at the midpoint, when the Pori side worked the puck well around the stretched defence and Peter Tiivola applied the finish.
It was 4-1 with still over four minutes of the second period to play, after Nestor Noiva was played through, and despite his shot being well saved, Tomi Karhunen could do nothing to prevent Patrik Juhola tucking away the rebound.
SaiPa had the better of the shot count in the final period, but any comeback ambitions were ended with just under eight minutes to play, when Jan Mikael Jarvinen finished in style to make it 5-1 after SaiPa again coughed up possession.
Last season's runners-up must now win two in a row to reach the semi-finals. Game six will be Assat's chance to win the series in front of their home fans on Saturday 11th April.
Saturday 11th, albeit on the road, will also be Ilves' first of two opportunities to seal their place in the semi-finals, after they won 2-1 at home to KalPa to go 3-2 ahead and end their two-game losing run.
Neither side took advantage of powerplays in a scoreless first period, but KalPa certainly took their chance four minutes into the second, as Lasse Lappalainen's centre was deflected in by Teemu Hartikainen.
For a long time, that looked like it was going to be enough for the Kuopio side, until 33 seconds of regulation time to play. Kasper Bjorkqvist had an effort saved, before Radek Kucerik did well to stop the attempted clearance, keep his composure and fire the equaliser past Talyn Boyko.
Kucerik's late heroics looked to be in vain when they were guilty of having too many men on the ice over 18 minutes into overtime, only for Lukas Jasek to intercept the puck at centre ice, race away from the scrambling KalPa defence and slot home the winning goal while shorthanded.
The reigning champions must now win twice in succession to maintain hopes of retaining the Liiga title.