One-goal wins are quickly becoming the hallmark of KuPS' season, and for the fourth time in six matches, the narrowest of victories would be enough to take them three points clear of SJK.
The contest was as tight as expected, with almost nothing to split the sides in terms of possession and shots, but the difference-maker arrived in first-half stoppage time.
On the second wave of attack, KuPS worked the ball out to Otto Ruoppi, whose square ball across the penalty box was helped on by Jaakko Oksanen to Agon Sadiku, who guided in his second of the season.
The home side had the better of the late chances, as SJK could not force a late equaliser and have now taken one point from their last two games after starting the season with four wins in a row.
SJK's loss leaves Inter Turku as the only other unbeaten side left this season, and they moved to within two points of the Seinajoki club with one fewer match played after a 1-1 draw with HJK.
Having finally got off the mark in the Veikkausliiga last time, Teemu Pukki made it two in two midway through the first half by timing his run to perfection and lashing a finish past goalkeeper Eetu Huuhtanen.
But 17 minutes from time, the Helsinki side threw away the win when a dreadful pass from goalkeeper Thijmen Nijhuis was intercepted by Loic Essomba, who rolled the ball into the empty net.
HJK are at least unbeaten in two matches, but have only four points and sit 10th of 12 teams.
AC Oulu are still bottom of the pile, but are now just a point behind HJK after ending their five-match losing run with a 3-1 win over 10-man KTP.
A terrible first half for KTP saw Joona Toivio score an own goal and Jiri Koski sent off, before Justin Rennicks and Marokhy Ndione eventually made the man advantage count in the final 12 minutes.
Armaan Wilson scored a consolation in added time for KTP, who drop to 11th.
After three successive wins, it's back-to-back defeats for Ilves, who remain fourth after losing 1-0 to VPS late on.
In the 93rd minute, Nicolas Fleuriau arrived at the back post to control a cross and bury a finish at the near post for VPS' second win of the campaign.
Jaro picked up their first points since matchday one with a 1-0 away win over Haka.
The new boys left things late as well - seconds after Haka's Nicolas Gianini Dantas was red carded in the 87th minute, Jaro capitalised when Severi Kahkonen's strike nestled in the bottom corner.
Finally, Gnistan earned their second win of the season and went above opponents Mariehamn into eighth place with a lightning start in a 2-0 home win.
A poacher's finish after three minutes and a low strike from the edge of the area after six minutes, both from Vertti Hanninen, sealed all three points.
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