Having entered the UEL following a disastrous second leg in their UEFA Champions League Second Qualifying Round tie with Kairat Almaty, KuPS will be well aware the tie is only at its midpoint, but they have put themselves in the driving seat with the home second leg to follow.
A foul on the edge of the box by Stefan Panic gave Jaakko Oksanen the chance to break the deadlock with 14 minutes played, and he made no mistake, as his free-kick went sailing into the top corner, giving Marko Maric no chance in the RFS goal.
In response, Mor Talla posed a threat from both wings for the Latvian champions, but a lack of end product and solid defending from the visitors limited RFS' attack.
Midway through the first half, things got better for KuPS after Mohamed Toure tried his luck with a curling effort from outside the area.
The strike was easily held by Maric, only for RFS to give the ball when attempting to play out from the back, allowing Toure to sneak in and clip the ball over the goalkeeper for 2-0.
But in the second half, RFS emerged looking like a whole new team and were soon creating chances through Davis Ikaunieks and Strahinja Rakic, who should have done better when the ball fell kindly for him in a crowded penalty area, only to blaze his effort over the bar.
Soon after, the Riga side finally took one of their chances - and in great style - carving the KuPS defence open with some excellent one-touch football in the middle, before Dmitrijs Zelenkovs played Barthelemy Diedhiou through, and he lofted the ball over the scrambling Johannes Kreidl.
With their tails up, the hosts put the KuPS defence under real pressure with many dangerous balls into the box, and though Diedhiou and Rakic were making their presence felt, Jarkko Wiss' men held onto their precious lead.
KuPS almost restored their two-goal lead with four minutes to play, but Samuli Miettinen's header went narrowly wide. The visitors had to survive a final onslaught in added time, but commanding goalkeeping from Kreidl allowed them to leave Latvia with a 2-1 win.
Getting the job done in next Thursday's second leg will see the Veikkausliiga champions progress to the Playoff Round, where they will face Fredrikstad of Norway or Midtjylland of Denmark.
Victory in the UEL Playoffs will see KuPS enter the League Phase of the UEL, while defeat will see them drop into the League Phase of the UEFA Conference League (UECL). Should RFS turn the tie around in Kuopio, KuPS will enter the UECL Playoff Round.