Man Utd hold Liverpool to hand Arsenal Premier League lead
The visitors dominated the majority of the game but needed a Mohamed Salah penalty six minutes from time to avoid a second defeat at Old Trafford in the space of three weeks.
The draw leaves Liverpool behind Premier League leaders Arsenal on goal difference and just one point ahead of Manchester City with seven games remaining in the tightest of title races.
Should the Reds fail to mark Jurgen Klopp' final season in charge by winning a 20th English top-flight title, to match United's record, they will look back on this day and wonder how two points got away.
For Liverpool, it was a similar story to their thrilling 4-3 FA Cup quarter-final defeat as they wasted a series of chances to extend their lead and ended up getting hit by the United sucker punch.
The home side did have the ball in the net after just 90 seconds but Alejandro Garnacho had ran offside well before receiving Fernandes' pass.
What followed was complete domination by Liverpool, who ended the first half with 15 shots to United's none.
Andre Onana produced one fine save to deny Dominik Szoboszlai but Liverpool were guilty of consistently letting the hosts off the hook.
Szoboszlai, Salah and Darwin Nunez were all wasteful in front of goal.
Liverpool did lead 1-0 at half-time as Luis Diaz fired in Darwin Nunez's header from a corner.
The pattern looked set to continue at the start of the second period as Nunez had a goalbound effort blocked.
But Liverpool were stunned as the title race took another twist out of nowhere on 50 minutes.
Jarell Quansah's slack pass went straight to Fernandes, who spotted Caoimhin Kelleher well off his line and lobbed the Irish goalkeeper from just inside the Liverpool half.
United suddenly had a fresh lease of life as Garnacho's deflected effort flew just over.
Mainoo brilliance
But Liverpool were still cutting the United defence open at will.
Nunez should have restored the lead when he fired across goal at the end of a lightning counter-attack from a United corner.
Instead, it was United who incredibly went in front 23 minutes from time.
Mainoo has enjoyed a meteoric rise this season and the 18-year-old produced the biggest moment of his career so far by curling into the top corner.
Klopp was incandescent on the touchline at his players for allowing Mainoo to get his shot off and immediately reacted by sending on Harvey Elliott and Cody Gakpo.
Elliott was the man felled by an Aaron Wan-Bissaka lunge for the penalty and Salah kept his cool to score for the 14th time in 15 games against United.
But Liverpool failed to find a winner to hand Arsenal the advantage in the title race thanks to the Gunners' nine-goal lead on goal difference.
A point also does little to aid United's hopes of qualifying for next season's Champions League.
Erik ten Hag's men are 11 points adrift of fourth-placed Aston Villa and eight behind Tottenham in fifth.