David Watson and Danny Armstrong scored early in either half for Kilmarnock to leave St Johnstone’s William Hill Premiership survival hopes hanging by a thread.
Watson’s fifth goal of the campaign after 12 minutes sent the visitors on the way to a comfortable 2-0 success.
Saints missed chances in the first period in the hunt for a leveller, but did not cause Killie any serious problems after Armstrong netted a 53rd-minute penalty.
Simo Valakari’s side once again failed to take advantage of a Ross County defeat.
They remain six points adrift at the bottom with three games to play and enter the last-chance saloon when hosting the Dingwall side next Saturday.
Valakari rolled the dice by giving Uche Ikpeazu his first start for the club.
The striker stepped up on two substitute outings since 11 months out with knee trouble.
Yet Kilmarnock were initially the dangerous team in the final third on and were rewarded with the opener.
Saints had multiple opportunities to get a foot in and prevent play from building.
However, Sven Sprangler slipped as Corrie Ndaba cut inside from the left and found Watson lurking on the edge of the penalty box.
Daniels Balodis stood off as Watson shifted the ball onto his right foot and sent a terrific effort into the far corner beyond goalkeeper Andy Fisher’s reach.
Armstrong’s miskick and a Fisher save from Stuart Findlay prevented Killie doubling their advantage.
Saints then took over and spurned opportunities to level. Makenzie Kirk collected on the turn from Ikpeazu 10 yards out, but lost his way and shot wide.
In the 34th minute, Kirk flicked on for Ikpeazu to stare down Kieran O’Hara from six yards.
However, on his first start since April 27 last year for Port Vale, the big forward fired straight at the goalkeeper.
Kilmarnock struck eight minutes into the second half to assume control.
Rory McKenzie, a half-time substitute for Swansea-bound Bobby Wales, won a penalty belatedly awarded by VAR.
The veteran tumbled under a lazy challenge from Victor Griffith. Only after Ikpeazu missed a headed chance at the other end did referee Iain Snedden take a trip to the monitor to the award the spot-kick.
Armstrong’s kick down the centre of the goal was almost blocked by the legs of Fisher.
However, the out-of-contract winger was able to lap up the celebrations of only his second goal of the season.
Ikpeazu was kept on the pitch much longer than expected and joined by Adama Sidibeh and Nicky Clark in search of a late spark.
Yet the Kilmarnock defence was entirely comfortable with what the home side mustered.
Derek McInnes’s deservedly moved on to 41 points, six ahead of the relegation play-off berth.
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