Ashton Gate hosted one of the biggest upsets in the recent history of the League Cup, as a last-gasp goal from Korey Smith not only put Bristol City into the last four, but knocked out current holders Manchester United.
The visitors had the better of the opportunities in the first-half as both Zlatan Ibrahimović and Marcus Rashford rattled the woodwork, but City showed they could be a danger as Daley Blind denied Smith with a last-ditch tackle.
City defied the odds as they took the lead with Joe Bryan's excellent 51st minute effort before Ibrahimović's quick equaliser, Romelu Lukaku tested Luke Steele on a few occasions but Smith's extra-time strike secured the historic win.
Showing their status early on
Anthony Martial curled a pearl of a cross in from the left which found the striker at the back post, he looked to square back into a dangerous but the deflection off Nathan Baker took it onto the crossbar.
Rashford went even closer minutes later as he was allowed too much space from the City defence, he decided to let rip from 20 yards with his low effort clattering the inside of the post.
Lee Johnson will have headed into this clash despite their opponent's status with some confidence considering their form, and showed their Premier League opponents that they meant business towards the end of the half.
Bobby Reid nodded the ball onto Smith who looked certain to find the opener with Sergio Romero to beat, but the Argentine was saved by the excellent last-ditch tackle of Blind.
David hitting Goliath with a mighty effort
City maintained their high energy levels as they came out for the second period, and punished The Red Devils in some style with their excellent opener.
It all came from Bryan who started the move with his flick over Blind before making a lung busting run towards the area, he got the ball back on the left-hand side but still had a lot to do but did brilliantly to fire it across Romero's goal into the top right-hand corner.
Quickly back in contention
If it hadn't before the party was certainly underway at Ashton Gate with their opener, but were dampened just as quick as it started as United grabbed an equaliser.
Ibrahimović stood over the free-kick just outside the area, a gaping hole in the City wall presented the Swede with a great opportunity to smash it into the bottom corner with the help of a deflection.
Sending the southwest into ecstacy
Mourinho decided to go for broke by lumping both Lukaku and Henrikh Mkhitaryan onto the field, Lukaku tested Steele on a few occasions with the best coming seven minutes from the end.
There was mayhem in the City area but the Belgian managed to get an effort through the legs of Aden Flint, the striker's effort looked destined for the bottom corner but Steele did well to get down and tip it wide.
Both sides threw everything but the kitchen sink at each other's goal as the clocked ticked into overtime, and it was City who secured the upset with their last-ditch winner.