Ayr boss David Hopkin claims referee Nick Walsh's "horrific" penalty decision cost his side dearly in their Premier Sports Cup last-16 defeat which went all the way to a penalty shootout at Somerset Park.
Tomi Adeloye fired the Honest Men ahead in the 55th minute but with 10 minutes remaining striker Nicky Clark levelled from the spot after Ayr defender Jack Baird, booked earlier by Walsh, was handed another yellow and a red for tangling with Tannadice defender Ryan Edwards inside the box.
The match went to extra-time and then the drama of penalties where Clark scored the decisive spot-kick for a 4-3 win.
Hopkin said: "Yes, it was tough to take.
"I said before the game that we were going to go at Dundee United and anybody at the game would have seen there was only one team that should have won the game.
"The decision to give the penalty was horrific and to send Jack off makes a fantastic up tie into a training session.
"It was extra soft. It was never a penalty. I saw it back.
"The ball is out the pitch, nobody claims for it and then the referee decides to get involved in the game because Dundee United didn't look like scoring and the only way it was going to come was a mistake or a poor decision and I stand by it, it was a really poor decision.
"We knew the longer the game went that we had a chance and I asked the players to dig in but it should never have gone that far.
"Ben Siegrist pulled off two world-class saves to keep Dundee United in the game.
"I don't think Aidan McAdams had a shot to save and we are out the cup."
Ayr skipper Aaron Muirhead, Sean McGinty and Nick McAllister scored their penalties but Tannadice keeper Siegrist saved from Patrick Ready and Jonathan Afolabi.
Tannadice substitute Marc McNulty, signed on a second loan period from Reading on Friday, skied the Taysiders' first penalty over the bar but Charlie Mulgrew, Peter Pawlett and Jamie Robson netted to take the visitors into Sunday's quarter-final draw.
Tannadice boss Tam Courts insisted he had complete confidence in Swiss keeper Siegrist.
He said: "Once we got to the penalty shoot-out that was probably the most comfortable and confident I have been in a penalty shoot-out in my managerial career.
"I just thought he enjoyed the centre stage and being the centre of attention and he was our match-winner in the end.
"It was a ding-dong cup affair.
"It probably went the way we expected, we knew it was going to be a competitive affair.
"When we got promoted out the Championship we didn't actually win at Somerset Park so we knew it was going to be a tough venue but I am really pleased the players found solutions and we are in the quarter-final and that is us one game from Hampden."
