A draw was all KuPS needed to reclaim first place on goal difference from Inter Turku, but they couldn't even manage that as they fell to a second defeat in five league games, as AC Oulu ended a run of three defeats in a row.
The only goal of the game came in the 28th minute when Oulu caught KuPS on the break and moved the ball wide to Joel Lotvonen on the right, and his low centre was guided into the bottom corner first time by Camilo Triana.
The Kuopio side were not overly creative in an even contest, but they wasted the perfect chance to level with a quarter of the match to go, as Tuomas Kaukua committed a foul in the area, but Piotr Parzyszek was unable to convert from the penalty spot.
From there, Oulu held on for a first win in five that takes them fifth and reignites their hopes of European football. KuPS good form over the summer has deserted them at the vital time - they have won just one of their last five in the league.
A Mads Borchers hat-trick helped HJK to a thumping victory at home to VPS which takes them to within four points of first place and five unbeaten in the league, while the Vaasa side drop to sixth having lost three in a row.
Teemu Pukki got the Helsinki side off the mark after just six minutes when reacting to a loose ball in the box, then Borchers added two before the break - the first fired into the bottom corner after intercepting a poor defensive header, and the second a close-range finish after the goalkeeper could only parry an effort.
The match ball had Borchers' name on it just eight minutes into the second half, when he shrugged off challenges before cruising into the area and finishing coolly. Leonel Montano then added a fifth just after the hour mark, before Alfie Cicale slotted home number six following Martin Kirilov's pass 10 minutes later.