Leipzig are on a 14-game unbeaten run which has seen them power into the last four of both the Europa League and German Cup, in which they host Union at home.
“We need a perfect performance to pull off a surprise,” admitted Union coach Urs Fischer, who said beating Leipzig at home would be a “shock” result.
His caution is understandable against a Leipzig side seeking a third German Cup final appearance in four seasons.
Leipzig’s French forward Christopher Nkunku is in outstanding form this season, scoring 30 goals including 17 in the Bundeliga, where he has provided 15 assists.
Since last December’s shock 2-1 home win over Manchester City in the Champions League group stages, Leipzig have recorded 15 wins, two draws and lost just twice.
When Domenico Tedesco took over from Jesse Marsch just after the City win, Leipzig were 11th in the Bundesliga.
They are now third in Germany’s top flight after Dominik Szoboszlai’s late goal on Sunday earned a 1-0 win at Leverkusen.
Leipzig bowed out of the Champions League despite downing Pep Guardiola’s City in the final group game, but powered into the Europa League semis, where they face Scottish side Rangers, by knocking out Real Sociedad and Atalanta.
To prepare for Wednesday’s German Cup semi-final, Union boss Fischer held a mini training camp on Easter Monday, where penalties were also practised.
As punishment, each spot kick which failed to hit the net meant paying for pizza or cleaning the changing rooms among the light-hearted punishments dished out.
‘Iron Union’, as they are nicknamed, have forged a reputation as a tough team to beat.
They earned a third straight win on Sunday as early goals by Taiwo Awoniyi and Grischa Proemel sealed a 2-0 victory over Eintracht Frankfurt, who had three days previously beaten Barcelona away at Camp Nou.
“We need a performance like Sunday’s over 90 minutes against Leipzig,” stressed Fischer.
Union are chasing only their second cup final appearance since losing 2-0 to Schalke in 2001.
RB Leipzig, who were founded in 2009, are seeking their third German Cup final appearance having lost to Bayern Munich in 2019, then Dortmund last season.
Second-tier Hamburg and Bundesliga side Freiburg clash in Tuesday’s other semi-final.
Despite a heavy fixture list due to competing in three competitions, Leipzig are staying fresh by using the breadth of their squad.
Tedesco made seven changes to Sunday’s starting side at Leverkusen from the team which won 2-0 at Atalanta three days previously when Nkunku scored both goals.
“That spirit is very important, so that we can make the changes without losing quality,” said Tedesco.