St Mirren, beaten finalists last season, paid the penalty for prioritising promotion here when they surrendered the lead and missed out on a quarter-final place.
Manager Jack Ross had made seven changes from the previous weekend and the weakened Championship leaders were undone by the League One pacesetters. The visitors saw an early attempt from Lewis Vaughan cleared off the line but the traffic mainly flowed in the other direction once Saints started moving through the gears.
Following a slow start, the hosts began to impose themselves on their opponents and only Rovers goalkeeper Graeme Smith and the woodwork stood between them and a comfortable half-time lead.
He produced fine saves to thwart Dale Hilson, Gary Irvine and Gavin Reilly before turning a blistering Ross Stewart effort on to the bar and over – referee Euan Anderson, unimpressive throughout, awarded a goal-kick. When Smith was finally beaten – by Jack Baird’s header from an Ian McShane corner – on the stroke of half-time, the crossbar came to his rescue. Saints would have cause to rue their profligacy.
Saints continued to dominate and they finally claimed the goal their pressure merited just after the hour when Reilly stole away from his marker to prod home an inviting cross from Hilson.
Unfortunately for the hosts, the old adage about teams being at their most vulnerable immediately after they have scored proved to be true here when Vaughan gave goalkeeper Ross Stewart no chance with an emphatic strike from the edge of the penalty area to notch his 16th goal of the season..
That was hard for the home side to take but things took a turn for the worse for them when Rovers went in front in their next attack.
David Mackay’s cross from the left was nodded into the path of Greig Spence by Vaughan and his strike partner prodded the ball behind Stewart from six yards.
Shell-shocked Saints crumbled and, when captain Adam Eckersley conceded possession 18 yards out, Bobby Barr took advantage by chipping the ball into the postage-stamp corner.