Amad scored in the final seconds of extra time to send Manchester United into the FA Cup semi-finals and end Liverpoolโs quadruple hopes.
All eyes were on Old Trafford as these great rivals met in a highly-anticipated clash that lived up to the hype on Sunday, swinging from one way to the other before a box-office conclusion.
Scott McTominay put United into an early lead, but Liverpool looked set to run amok after Alexis Mac Allister and Mohamed Salah scored late in the first half to put Jurgen Klopp's men in control.
Under pressure Erik ten Hag's side offered little in response, but substitute Antony managed to level against the run of play in the 87th minute.
Marcus Rashford blew a gilt-edged opportunity to win the quarter-final with the final kick of regulation time, with the roller-coaster ride continuing in extra time.
Liverpool sub Harvey Elliott landed a 105th-minute body blow but, in front of United fan Tyson Fury, the hosts picked themselves off the canvas.
Rashford drew United level in the 112th minute and Diallo scored on the break in added time at the end of extra time, shaking Old Trafford to its foundations.
The 21-year-old was sent off after receiving a second booking for taking his shirt off during the ear-shattering celebrations on a day that seals the winger's place in United folklore.
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โ ITV Football (@itvfootball) March 17, 2024
Amad Diallo sends @ManUtd into the @EmiratesFACup semi-finals!#EmiratesFACup | #MUNLIV pic.twitter.com/2ZuomoLF9n
Sunday's helter-skelter clash began with a surprisingly bright United start, with returning Aaron Wan-Bissaka and Rashford having early efforts.
It was not one-way traffic and Salah hit a volley back across goal, but United turned their early promise into a 10th-minute opener.
Alejandro Garnacho shot instead of passing but his poor decision paid off as Caoimhin Kelleher's parry sent the ball looping for McTominay to stretch and poke home.
The midfielder should have doubled United's lead in the 35th minute, but he struck straight at Liverpool's goalkeeper after Kobbie Mainoo's majestic footwork and a Rashford cutback.
Liverpool took things up a gear after that. Andre Onana denied Luis Diaz and Wataru Endo soon rolled past United's goalkeeper, only for the flag to be raised for an offside in the build-up.
Increasingly ragged United failed to heed those warning shots and were punished in the 44th minute.
Liverpool centre-back Jarell Quansah was all-too-easily able to maraud forwards, resulting in Darwin Nunez teeing up Mac Allister to take a touch and hit an effort that deflected in off Mainoo. The 9,000 away fans erupted and they were back on their feet again in stoppage time.
Bruno Fernandes' claims for a foul by Joe Gomez fell on deaf ears and he sent in a cross, leading to Nunez seeing a shot saved and Salah slamming in the loose ball.
Liverpool smelt blood as United left gaping holes and Onana stopped a Nunez effort from creeping in. The striker and Dominik Szoboszlai had efforts before Mac Allister's free-kick clipped the roof of the net as some United fans sung offensive chants. Onana stopped Nunez brilliantly as Liverpool looked set to cruise to a straightforward win.
But Antony, so often underwhelming since joining in 2022, had other ideas, collecting a loose ball in the box, spinning and scoring from a low shot with his weaker right foot.
Old Trafford exploded and United, having survived an Elliott cross-shot hitting a post after Onana went walkabouts, should have won it with the final kick.
Christian Eriksen's lifted ball put Rashford behind with just Kelleher to beat, but he followed a cushioned touch with a low shot just wide. There was a hint of offside but all inside Old Trafford were stunned and both sides had chances when extra time got under way.
The first period was petering out until Elliott tried his luck from 25 yards, with his strike deflecting off Eriksen's boot, going through Harry Maguire's legs and beating Onana.
Both teams were exhausted but United, playing in a hodge-podge formation, continued to fight.
Maguire saw a shot stopped and in the 112th minute Diallo cut out a tired Nunez pass, starting an attack that saw McTominay put through Rashford to beat Kelleher.
McTominay directed wide and United were not to be denied a winner, clearing a Liverpool corner and breaking at pace through Garnacho and Diallo.
The latter appeared to have taken it too wide but beat Kelleher with a low shot, sparking pandemonium and getting sent off for his celebration.
