The teams who finished fifth to 12th in the regular season began their best-of-five series with the goal of securing a place in the Quarter-finals
The four winners will face Lukko, Ilves, KalPa or IFK Helsinki, who occupied the top four places in the standings after 60 rounds of matches.
Two Otto Salin goals were enough for TPS Turku to cause the first big upset of the playoffs, as the 12th-placed side won 2-1 away in Lappeenranta against SaiPa.
Salin struck four minutes into the game with a long-range strike and in the final two minutes of the second period to give his side the lead again, after Janne Naukkarinen had equalised for SaiPa after 12 minutes.
Not to be outdone, Vaasan Sport also sprung a surprise with a 3-1 win away at KooKoo.
After a goalless first period, Jimi Suomi put the home side ahead early in period two.
But any hopes the sixth-placed side had of strolling to a series lead were put to bed by Viljami Niemenen and Simon Hjalmarsson, who turned the game on its head going into the final 20 minutes.
Using a powerplay to full effect three minutes into the third, Sport stretched the home defence with a neat team move, finished off by Cole Fonstad, to seal the win.
Assat had no such home struggles, seeing off a spirited Hameenlinna 3-2 in Pori.
The home side took the lead in the eighth minute when Jonne Tammela was quickest to react to a loose puck after sustained pressure from the hosts.
But the scores were level again by the end of the first period, when Nikkila Petteri took advantage of a powerplay.
A clever Roope Tajala flick from a tight angle put Assat back in front 12 minutes into the second, but Hameenlinna hit back again, this time through Mikka Kukkonen within three minutes.
With seconds ticking down at the end of the second period, Valtteri Viljanen tried his luck from just inside the blue line and his shot sneaked past the goaltender, Kim Saarinen.
But the visitors could not find an equaliser for a third time, allowing Assat to take the lead in the series.
Finally, the match between the teams who finished eighth and ninth in the regular season was as close as expected, but Kiekko-Espoo prevailed with a 1-0 win over Tappara.
In the ninth minute, Cameron Hillis and Arttu Tuomaala took advantage of a powerplay to set up Santeri Virtanen to score from close range.
The teams have little time to reset, as game two of each series takes place tomorrow.