A Scott Fox penalty save spared Ross County late on before a Jamie Lindsay header won the battle of the two Championship title favourites for the hosts.
The Highlanders’ keeper denied United’s Nicky Clark from the spot with a double save with nine minutes left. Four minutes later, the Staggies snatched all three points from the jaws of defeat with a superbly-worked clincher from Lindsay to give County maximum points after their two matches in Group A.
Two teams with promotion aspirations took time to size each other up but United threatened first after 15 minutes. Paul McMullan’s free-kick from the right found summer capture Frederic Frans towering above the County defence but looping his header over the bar from eight yards.
Two minutes later, County mustered a similar chance from Josh Mullin’s free-kick but Sean Kelly’s leap ended in similar fashion to Frans’ effort.
County gradually gained the upper hand and went close after 24 minutes. Michael Gardyne’s trickery created space on the left and Billy Mckay knocked down the County winger’s cross just inside the area.
Lindsay let rip but keeper Matej Rakovan turned the fierce attempt beyond his right-hand post.
County defender Liam Fontaine then came off worst after catching Rakovan with a stray arm after chasing down a through ball and had to be replaced by Callum Morris.
Both teams saw further attempts cleared off the line before the break.Keith Watson’s header was knocked clear by United’s McManus while, at the other end, Adam Barton’s aerial attempt was nodded to safety by Watson.
The stalemate continued deep into the second half. But the hosts almost carved the Tangerines open after 63 minutes with Kelly’s cross from the left beckoning for Declan McManus before a timely clearing challenge from Frans spared United.
United substitute Declan Glass added some energy for the visitors and broke immediately, but his shot was parried by home keeper Scott Fox.
Home tempo was building, with Declan McManus denied twice on the angle by keeper Rakovan, either side of an attempt by Gardyne that went wide.
But, with nine minutes left, United blew a match-winning chance. Gardyne was adjudged to have tripped Sam Stanton just inside the penalty area.
But keeper Fox read Clark’s penalty brilliantly, diving to save to his right before reacting to send the United striker’s strike on the rebound over the bar.
Four minutes later, County substitute Ryan Dow wheeled clear on the right and sent in a perfect cross for Lindsay to net with a downward header from six yards.