Billy McKay hit a double to earn a 2-1 win over St Johnstone and continue Ross County's perfect Betfred Cup record.
Scott Tanser gave the hosts an early lead from the penalty spot but their second defeat means they will crash out of the tournament before the knockout fixtures begin.
Saints had six players back from injury following their midweek defeat by Montrose and Callum Hendry shook off a knock to start on the bench.
They started well and Tom Grivosti conceded a penalty with a foul on Chris Kane. Tanser confidently converted in the eighth minute.
The hosts continued to press. Danny Swanson shot over from Murray Davidson's cutback and Michael O'Halloran hit the post from a tight angle with Keith Watson guarding the net after beating Ross Laidlaw to Swanson's through ball.
But County got a lifeline in the 30th minute after Jason Kerr was controversially ruled to have barged over Ross Stewart in the box by referee Bobby Madden. Zander Clark saved McKay's penalty but the striker quickly converted the rebound.
Saints soon had a spot-kick appeal rejected when Grivosti appeared to pull Swanson's shirt and the visitors took the lead in the dying seconds of the half.
County won two headers after goalkeeper Laidlaw's kick-out before the ball broke off to Blair Spittal. The midfielder's through ball found the frontman, who got away from Steven Anderson and finished well.
The Ladbrokes Premiership newcomers set about extending their lead after the interval.
Clark made a double save from Ross Stewart and Sean Kelly before tipping over Iain Vigurs' free-kick. Joe Chalmers then came close from 18 yards and Josh Mullin headed into the side-netting after a brilliant move.
St Johnstone put pressure on in the final 20 minutes but Kane missed a glorious chance after being sent clean through by substitute Hendry.
The striker hit his shot straight at Laidlaw, prompting Tommy Wright to boot a bag in frustration before taking Kane off seconds later.
