Just one point separates them in the Premier League table, and both teams go into this repeat of the 2018 final off the back of crucial victories at the weekend.
It is hard to take one's eyes off a thrilling top-four race in the Premier League table at the moment, with Chelsea in third and Manchester United in fifth separated by just one point, but that is what both clubs must do this weekend with Wembley awaiting.
The lure of an FA Cup semi-final will make it easier to do so, with the trophy playing a major part in the history of both of these clubs and it remaining their most realistic chance of silverware this season - despite both still being involved in European competition too.
United and Chelsea have lifted the FA Cup 20 times between them, with United looking to do so for a record-equalling 13th time this season and also bidding for a record 21st appearance in the final.
The impressive pedigree of these two clubs in this competition does not stop there; this is United's 30th appearance in the semi-finals, while Chelsea have made the last four for the 10th time since the turn of the century - both records shared by Arsenal.
Both managers know all about success in the FA Cup too, with Frank Lampard winning it four times as a player and Ole Gunnar Solskjaer twice; both are now looking for their first major trophy as a manager.
It is Solskjaer's side who will go into the game as favourites for many given their recent form, which has seen them win six and draw two of their eight matches since the restart.
United's unbeaten run now stretches to 19 games across all competitions, which is their longest such streak in almost a decade - last doing so in November 2010 when Sir Alex Ferguson's side went 29 games without tasting defeat.
It is the manner in which they have picked up those results which will please Solskjaer the most, though; after years of accusations of being stale and slow, the partnerships between Bruno Fernandes, Marcus Rashford, Anthony Martial, Mason Greenwood and Paul Pogba have provided a welcome and exciting breath of fresh air to the United attack.
Fernandes was the only one of that quintet to start in the quarter-final against Norwich City, and it was therefore perhaps no surprise to see them struggle in comparison to their Premier League form, needing a 118th-minute extra-time winner from Harry Maguire to overcome the Premier League's bottom side.
Todd Cantwell's goal in that match was the first United had conceded in 448 minutes of FA Cup action this season, though, having kept clean sheets in victories over Wolverhampton Wanderers - via a replay - Tranmere Rovers and Derby County before that.
In general, United's defensive record has significantly improved since their last defeat in January, keeping clean sheets in 13 of their 19-match unbeaten run including Thursday night's crucial 2-0 victory over Crystal Palace.
Questions over VAR once again arose in that match with Palace denied a penalty, but goals from Rashford and Martial ensured that United remain very much in the mix for a top-four finish, only being kept out of the Champions League places by goal difference.
Such is the tight nature of that battle that every remaining game this season is likely to feel like a knockout game for both of these two teams, with Chelsea currently in pole position to claim one of the remaining spots but still with no room for error if Leicester City and United keep winning.
The Blues made a vital return to winning ways over relegated Norwich on Tuesday night as Olivier Giroud fired them to a 1-0 victory, bouncing back from a hugely disappointing 3-0 defeat at Sheffield United last weekend.
Such inconsistency has plagued Chelsea all season in truth, and with matches away to champions Liverpool and at home to Wolves still to come United will be optimistic that Sunday's opponents will drop points between now and the end of the campaign.
Inconsistent is not a word you could use to describe Chelsea in the FA Cup, though; they have won 11 of their last 12 outings in this competition stretching back to their defeat in the 2017 final, including lifting the trophy by beating United in the final two years ago.
As it happens, United are the only team to have beaten Chelsea in that run - knocking the Blues out in the fifth round last season - but this is a competition that they have taken seriously in the Roman Abramovich era, winning it five times in the last 13 years.
So far this season Chelsea have overcome Nottingham Forest, Hull City, Liverpool and Leicester to reach the semi-finals, conceding only one goal along the way.
It took a Ross Barkley strike to see them past the Foxes in the last round, with the midfielder having now accounted for three of the seven FA Cup goals they have scored this season.
Lampard was clearly unhappy with his side's first-half performance in that match, though, making three half-time subs to change the game in a sign that FA Cup glory does still mean a lot to him and the club despite the notable distraction of such a tight battle to reach the Champions League.
Man Utd FA Cup form: DWWWW
Man Utd form (all competitions): WWWWDW
Chelsea FA Cup form: WWWW
Chelsea form (all competitions): WLWWLW
Shaw twisted his ankle against Southampton and, while he avoided any damage to his ligaments, the nature of that injury is likely to take longer to heal than Williams's cut to his face.
The latter is not thought to be a concussion concern, meaning that he should be available to return for this match after Solskjaer handed a rare start to Timothy Fosu-Mensah against Crystal Palace.
Diogo Dalot's omission from that match set tongues wagging about his future at the club, but the FA Cup could give him and several other fringe players a chance to start with Solskjaer likely to rest many of the forward players that have led their Premier League revival.
Odion Ighalo is one of those who will be pushing for a start, having scored every time he has been named in the starting XI for the club.
Lampard must also decide to what extent he wants to rotate players in order to keep key men fresh for the top-four race despite having two days more rest than United, but he will still be without injured midfielders N'Golo Kante and Billy Gilmour.
Man Utd possible starting lineup:
Romero; Dalot, Bailly, Maguire, Williams; Matic, Fred; James, Pogba, Rashford; Ighalo
Chelsea possible starting lineup:
Caballero; James, Tomori, Christensen, Emerson; Barkley, Jorginho, Mount; Loftus-Cheek, Abraham, Hudson-Odoi
Head To Head
This will be the fourth meeting between these two sides this season, and Manchester United have won all of the previous three - home and away in the Premier League by a 6-0 aggregate scoreline and a 2-1 triumph to knock the Blues out of the EFL Cup.
Indeed, United are now unbeaten in their last six games against Chelsea across all competitions, stretching back to the FA Cup final between the two sides in 2018.
Chelsea have won four of the last five FA Cup ties between these two clubs, but the overall record in this competition stands at nine wins for United compared to five for Chelsea.
We say: Man Utd 2-1 Chelsea
This is a mouth-watering battle which is very difficult to call - even more so given that both managers are expected to make changes.
Chelsea have had a couple of extra days to prepare, but the formbook points towards a United win and that is the side we have fallen on, with their current run just about giving them the edge - although this is a contest which could quite easily go either way.