The Kuopio side had picked up just one point from their previous four league outings, albeit all against fellow top flights sides, so there was optimism they could get back to winning ways against Mariehamn, who began the round ninth and without a win in three.
However, they were powerless to stop the rot when, 11 minutes from time, Emmanuel Okereke popped up with the winning goal, his second in as many games, as Mariehamn got revenge on the side who beat them 4-1 earlier this season.
Gary Williams' side move up to eighth on 14 points, now only nine points shy of KuPS, who drop two places to third.
One of the two to overtake KuPS are Inter Turku, who romped to a 5-0 win at home to strugglers KTP on Thursday to go two points clear at the top and take their unbeaten run this season to 11 games.
Loic Essomba broke the deadlock for the hosts, before Jean Botue provided some breathing space just before the break. Essomba turned provider for Dimitri Legbo just after the hour mark, then Jussi Niska added number four, seven minutes from time.
In the 93rd minute, Jasse Tuominen completed the scoring for Turku, who have won four of their last five in the league, while KTP stay 11th with one win all season.
HJK are now second, ahead of KuPS on goal difference, after they beat Haka 3-1 in Helsinki, going 11 matches unbeaten in all competitions.
Not for the first time, it was Alexander Ring and Teemu Pukki who did the damage early on - Pukki set up Ring for the opener after 13 minutes, before the former Premier League man made it 2-0 10 minutes later.
Jean-Pierre Da Sylva quickly pulled one back for Haka, only for HJK to make sure of the points just before the hour, through Benji Michel, with Ring providing the assist. Haka slip back to ninth with their first loss in four games.
After a woeful few weeks, SJK are now three matches unbeaten and have crushed Ilves' winning run with a 3-2 victory in Tampere.
Oskari Vaisto scored the only goal of the first half for the away side, before Ilves hit back through Joona Veteli. A Rasmus Karjalainen penalty restored the SJK lead, which they extended in the 90th minute through Elias Mastokangas.
Vincent Ulundu grabbed a consolation, but now only one point separates Ilves in fourth and SJK in fifth, after a midweek that could have seen the Lynx go level on points with Inter Turku.
Elsewhere, AC Oulu went 1-0 up through a Niklas Jokelainen strike despite being down to 10 men after Abu Dumbuya was dismissed in the first half, but the bottom side couldn't hold on, as Jim Myrevik and late substitute Rudi Vikstrom turned the match around for Jaro to win 2-1.
Finally, the mid-table battle between VPS and Gnistan ended 0-0 in Vaasa.