The veteran goalkeeper Thomson, 38, saved twice in a penalty shoot-out at Tannadice as Jim Duffy's First Division side knocked out their Premier Division neighbours. United led through Owen Coyle but Jim Hamilton scored twice, once from the spot, before Gary McSwegan's equaliser ensured a shoot-out in a tie that was deadlocked at 1-1 after 90 minutes and 2-2 after extra-time.
"I'll get the credit for saving two in the shoot-out but I hold my hands up over the first goal, I sold that one," admitted Thomson, who blocked penalties from Coyle and McSwegan.
While Duffy singled out Thomson for special mention following the former United man's vital saves in the shoot-out, he was not prepared to miss out any of his entire side. "Everyone was a real hero," he said. "There are very few games in which a manager emerges with nothing to complain about, but that was one of them."
The United manager, Billy Kirkwood, was not so happy. "My players let themselves and their team-mates down. Every one of them placed pressures on their team-mates at some point in that tie," he said.
Aberdeen, the Scottish Coca-Cola Cup holders, survived a scare in an incredible match at Cappielow against Allan McGraw's First Division leaders, Morton.
Billy Dodds scored twice to have the Dons apparently coasting until the referee, Mike McCurry, awarded Morton a controversial penalty. Derek Lilley converted and, after John Anderson equalised, Lilley fired the underdogs ahead.
Dodds spared Aberdeen's blushes to score his hat-trick with a late spot- kick and ensure another half hour. Extra time belonged to Dean Windass, who scored four goals to see Roy Aitken's men safely through.
Hearts edged through in Perth with a 3-1 win after extra-time against St Johnstone at McDiarmid Park.
Colin Cameron and George O'Boyle traded goals before Darren Beckford and John Robertson put Hearts into the last eight. Robertson needed six stitches inserted in a head wound after being taken off on a stretcher during extra-time.
Hibernian eased the pressure on their manager, Alex Miller, as they came through against the Third Division leaders, Albion Rovers, at Cliftonhill.
Keith Wright put the Premier Division visitors ahead at Coatbridge after 18 minutes and a spectacular second-half effort from the midfielder Pat McGinlay wrapped up victory for the Easter Road outfit.
Partick Thistle also went through to the quarter-finals thanks to a 1- 0 win over the cup specialists, Airdrie, at Firhill Park. Gareth Evans scored the vital goal in the first-half.Reuse content