A hat-trick from Liam Boyce secured a 3-0 win for Ross County at St Mirren Park, with the visitors moving 16 points clear of their fellow strugglers with just five games remaining this season.
With the league set to split into a championship group and relegation group on April 22 when all teams have played 33 matches, Jim McIntyre's Ross County can only be relegated if they drop into 11th position, but they would still have to lose a two-legged play-off against the Scottish Championship's second-best club.
Northern Ireland striker Boyce picked up where he left off by scoring a treble six days after he notched an 84th-minute winner against St Johnstone last Tuesday.
Boyce struck his first goal on the half-hour mark, converting inside the penalty area after two of his team-mates were denied by St Mirren's desperate defending.
St Mirren's hopes of claiming vital points against a relegation-threatened rival were dealt a massive blow three minutes into the second half when Viktor Genev was shown a straight red card for fouling Boyce when he was the last defender.
It was 2-0 in the 75th minute when Boyce scored from Michael Gardyne's cross, while the 24-year-old forward completed his hat-trick in the second minute of stoppage time after Jackson Irvine and Tony Dingwall combined.
The loss - St Mirren's fifth in a row - leaves Gary Teale's side on 21 points and ensures they cannot catch Ross County with their five remaining games, while Motherwell (31) currently occupy 11th, six points adrift of McIntyre's men.
Ross County need a maximum of 10 points in their five matches in the relegation group to secure top-flight football for next term.
