Two goals from Nathaniel Chalobah and one from Bobby Devyne gave Chelsea's Under 18s a lead to take to Old Trafford for the semi-final second leg.
Chalobah's strikes took the schoolboy's total in the cup run to four and gave the young Blues a two-goal lead at the end of a first-half they dominated. But Manchester United, who came from two goals behind to beat Liverpool at Anfield in the quarter-final, improved greatly in the second period.
The visitors made it 2-1 before Devyne scored his second Youth Cup goal on the break, an advantage that was cut again as Man United showed aerial power to score and keep the tie well-balanced for the second leg. Away goals do not count double in this competition.
England Under 17 captain Chalobah had moved from midfield into central defence to replace suspended captain Daniel Pappoe and schoolboy Adam Nditi came into midfield for injured Billy Clifford, with fit-again Rohan Ince (pictured below) also returning to the midfield.

Josh McEachran captained the side in the absence of Pappoe who will be available for the second leg.
The Chelsea midfield quickly got on top, George Saville (below) winning early challenges in the Blues half and the more-forward McEachran putting early pressure on the Man United defence.
On three minutes came the first chance, Milan Lalkovic stooping to head a Todd Kane cross but it was close enough to Sam Johnstone that the Man United keeper was able to make a good save.

McEachran very nearly put the fleet-footed Lalkovic clean through on goal on 12 minutes with an early pass from deep in his own half but Johnstone was quickly out of his area to hack clear, then Nditi wormed his way into the Man United box but couldn't put any power behind his shot.
Will Keane had the visitors' first genuine effort on 14 minutes but Jamal Blackman held a 20-yard shot with safe hands.
Chelsea attacked again, Aziz Deen-Conteh forward from left-back skipping past Man United full-back Michael Keane with ease and delivering a low cross that neither Bobby Devyne nor Lalkovic could capitalise upon.
There was plenty of strong running from Chelsea's main attacking three and Devyne won a free-kick that Ince hammered into the wall before Lalkovic again forced Johnstone into a good save, this time with an angled shot.
From the corner that resulted, taken by McEachran, Chelsea took the lead. This time the Man United keeper was found wanting as he failed to catch under pressure and as the visitors' defence dithered, Chalobah rammed the ball in from close range with 29 minutes played.
On 37 minutes Chelsea got away with our own defensive slip as the ball bounced back off Chalobah's head into the space between him and keeper Blackman but Will Keane kicked air rather than the bouncing ball and the danger was cleared.
Dermot Drummy will have been delighted with some of the football his side were playing, as must have been Roman Abramovich who was present inside the stadium, and an excellent passing move, with impressive Saville involved, won a corner three minutes before the break.
Saville took it and Chalobah met the ball with a header so perfect that neither the keeper nor the man on the far post could prevent it finding the top corner.

The young Blues had a 2-0 lead to take into the break, although it needed a save from Blackman to keep Man United out with the last action before the half-time whistle.
Blackman was called into action again 90 seconds after the restart, diving to save after Gyliano van Velzen drilled a shot through a tight gap. It was very good stop from the 6ft 5in keeper.
Man United were enjoying their best spell, Chalobah this time excelling in his defensive duties with some vital and committed interventions.
On 55 minutes Blackman tipped over a 30-yard shot from Paul Pogba but the visitors scored from the corner which skimmed over a crowd in the centre and fell for Jesse Lingard to hook the ball into the net.
Drummy reacted to his side losing control of the game by making a 62nd minute substitution, Archange Nkumu coming on and going into midfield with Ince making way. Lewis Baker replaced Nditi eight minutes later with Chelsea still trying to rediscover the flowing football of the first half.
Deen-Conteh had been a problem for the Man United defence throughout the game and showing the power that had been so prominent in the winning campaign last season, he charged outside Tom Thorpe and squared perfectly to miss the keeper but find Devyne who swept the ball inside the far post.

The restored two-goal lead lasted less than five minutes. The tall Pogba rose highest to meet a cross from the right by Cole and head across Blackman for a good goal. Shortly before that, the Chelsea keeper had made an excellent point-blank-range save from Will Keane.
The Man United threat continued and Lingard shot just wide on 80 minutes but in energy-sapping heat, the final 10 minutes were played out largely without incident at either end of the pitch.
'It was a great game of academy football, a fantastic, open match that I am sure people enjoyed watching,' said youth team manager Dermot Drummy.
'I thought we had the first half and could have had a couple more goals but you have to give credit to Man United, they came back really strongly. We performed fantastically in terms of mental toughness because we had a lot of young boys out there and they were playing up against a really strong side.'
On Nathaniel Chalobah's goal habit in this Youth Cup campaign, he said:
'He is almost like an attacking central defender! He can bring the ball out and play but he was also putting in block tackles. He has two more goals and he is still an Under 16 so he is a super talent.
'Aziz Deen-Conteh did well to make the third goal for Bobby Devyne. We wanted to get behind the Man United defence so they were facing their own goal.
'The heat did have an effect. Your brain goes in those temperatures but you have to think quickly and try to control the tempo of the game. We did it first half and they did it second half.
'A lot of these players will be involved in the reserves on Thursday, then we will rest them at the weekend and then travel up to Manchester for what I am sure will be a very exciting game. We are not going to go there to defend. You can't defend for 90 minutes at Old Trafford.'
The second leg is on Wednesday 20 April and is a game that can be watched live on Chelsea TV and Chelsea TV Online. The reserves also play away to Man United on Monday 18 April, again live on Chelsea TV.

Chelsea (4-3-3): Jamal Blackman; Todd Kane, Tomas Kalas, Nathaniel Chalobah, Aziz Deen-Conteh; Rohan Ince (Archange Nkumu 62), Josh McEachran, George Saville; Bobby Devyne (Ben Sampayo 83), Milan Lalkovic, Adam Nditi.

Man United (4-1-4-1): Sam Johnstone; Michael Keane, Tom Thorpe, Michele Fornasier, Sean McGinty; Larnell Cole; Jesse Lingard (Tom Lawrence 83), Paul Pogba, Ryan Tunnicliffe, Guliano van Velzen; Will Keane (Daehli Mats 88).