Three defeats in their previous four had seen Ilves lose touch with the top of the standings in recent weeks, and few expected their form to improve against the side who had already opened a seven-point lead.
But KuPS' recent scoring form, which had seen them notch three in their previous two outings, deserted them as neither side mustered an effort on target in the first half, though the hosts showed the better attacking endeavour.
And they were rewarded 10 minutes after half-time, as continued pressure on the visitors' box resulted in Oiva Jukkola trying his luck from the edge of the area and nestling the ball in the bottom corner.
With 20 minutes to play, the Lynx doubled their lead when a deep corner was nodded back into the danger area and eventually dropped to Matias Rale, who swivelled and caught the half-volley perfectly.
The perfect night for Ilves was completed two minutes from time - having broken out of their own half, the ball was worked to Marius Soderback, who drifted across the area and guided the ball past Johannes Kreidl.
The win takes Ilves to third in the table, level on 15 points with Inter Turku, the last unbeaten side in the Finnish top flight.
A first league defeat since their previous trip to Tammelan Stadion in September keeps KuPS seven points clear, though they have played twice more than Inter Turku.
HJK began the season with three defeats, and SJK with four victories, yet a month later only one point separates them in the table after the Helsinki club won 4-2 on Tuesday night.
The home side blitzed into a 3-0 lead after just 35 minutes, with Teemu Pukki living up to his stardom for HJK. In the eighth minute, the striker squared the ball across the edge of the area for Santeri Hostikka to guide beyond the goalkeeper with aplomb.
After 15 minutes, Pukki got his second assist by driving into SJK territory and digging out a ball over the top for Alexander Ring to loft over the goalkeeper for 2-0.
And the 35-year-old got himself on the scoresheet with HJK's third - his side won possession back high up the pitch before he latched onto Ring's through ball, and though goalkeeper Roope Paunio got a strong hand to it, he couldn't prevent the ball finding its way into the net.
Pukki failed to register a goal involvement in HJK's three defeats at the start of the season, but has at least one in each of the five matches the Klubi have gone unbeaten since then.
A spectacular own goal from Aniis Machaal made it 4-0 after the break, before a Rasmus Karjalainen penalty and a Valentin Roman Gasc tap-in added consolations for the Seinajoki side, who sit fourth with just two points taken from their last five games.
HJK move into fifth and see their record under interim manager Miika Nuutinen stretch to three wins from three.