Lucas Vazquez sent Madrid into a sixth-minute lead in the first leg earlier this month, before Malcom registered for the Catalan giants in the second period to leave the last-four tie delicately poised.
Barca landed their 30th Copa del Rey crown last season and have lifted the trophy in each of the last four campaigns, but Madrid have triumphed just twice in the competition since 1993.
It has not been the go-to competition for Madrid like it has been for Barca, but it might well represent the club's best chance of silverware this season as they are nine points off the pace in La Liga and face a tough battle to retain the Champions League.
Wednesday night will bring the 240th competitive El Clasico. Madrid lead the head-to-head 95 wins to Barcelona's 93, with the remaining 51 games finishing level.
Here, Sports Mole has selected the best team from both squads, using only players available for the second leg of the semi-final this week.
There is no question that Jasper Cillessen would have started this match for Barca had he been fit, but the Dutchman will miss out once again with a muscular injury. As a result, Marc-Andre ter Stegen will play in the Copa del Rey for just the second time this season. Madrid have two excellent goalkeepers in the shape of Thibaut Courtois and Keylor Navas - the latter of which will start this match - but Ter Stegen has taken his performances to a new level this season and deserves his spot in this XI.
Marcelo's struggles this season means that left-back is straightforward to pick, with Jordi Alba enjoying an excellent campaign - contributing an incredible 14 assists in all competitions. Meanwhile, Los Blancos defender Dani Carvajal gets the nod over Barcelona pair Sergi Roberto and Nelson Semedo on the opposite side as there is arguably not a better right-back in Europe.
Raphael Varane is one of the outstanding centre-backs in the current game, but his lack of consistency this season means that the Frenchman misses out in this team. Clement Lenglet has played a lot of football for Barca this term due to the absence of Samuel Umtiti, but Gerard Pique has again been his team's outstanding central defender and sails into this combined XI.
Alongside Pique sits Sergio Ramos. Back from a domestic suspension, the 32-year-old has scored 11 times in all competitions for Madrid this term, including three in the Copa del Rey. Ramos still has his critics and perhaps always will, but he is an inspirational figure for Los Blancos. In this XI - which would have a fairly good chance of being successful - the Spaniard would take the penalties and indeed sport the armband.
Holding the midfield in a 4-3-3 formation would be Sergio Busquets. It was a straight fight between Busquets and Casemiro for this position, but the Spaniard is the superior footballer. The 30-year-old has not scored in 36 appearances in all competitions this season, but is an absolute genius in the middle of the park. Barca teammate Ivan Rakitic also makes the team, with the Croatian boasting four goals and seven assists in all competitions this season. There has been talk of a summer exit in recent weeks, but Rakitic is still a hugely important cog in the Barca machine.
It would be fair to say that Toni Kroos has not been at his best for Madrid this season and the Germany international misses out. Luka Modric does make it, but he is the only Los Blancos figure in the central three. Modric, who starred alongside Rakitic at the 2018 World Cup, found it difficult to show his best form in the first half of the 2018-19 campaign, but has been excellent since the turn of the year.
The easiest decision in the world is to select Lionel Messi as one of the front three. The 31-year-old has 33 goals in 32 appearances this season and struck a hat-trick against Sevilla at the weekend. Barca have not been at their best for a number of weeks, but continue to be dragged through matches by the stunning Argentine.
It was Karim Benzema versus Luis Suarez for the centre-forward position and the former just takes it. Benzema has out-scored Suarez 20 goals to 17 this season, while it would be fair to say that Barca's striker has been far from his best in recent weeks. Benzema, on the other hand, is arguably having his finest ever season in a Los Blancos shirt, particularly in terms of overall contribution.
Gareth Bale's injury problems and lack of starts means that it is easy to leave the Welshman out, while Ousmane Dembele has also had his fitness issues this term and is another player to just miss a spot in the XI. Philippe Coutinho, though, is miles off due to his struggles for the club this term.
Marco Asensio has also found it difficult for Madrid this season, but Vinicius Junior has starred for the European champions and gains the final spot in this XI. It is true that the Brazilian still has a lot to work on, but at 18 years old, he has seven goals and 13 assists to his name this season. The fact that seven of those assists have come in the Copa del Rey means that it is impossible to leave the South American out.