The two sides drew 3-3 in the first leg in Germany last week, with Urs Fischer's side coming from behind on three occasions to bring the tie level.
Match preview
Indeed, Union SG boss Karel Geraerts would have been left frustrated by his side squandering a lead three times at Stadion An der Alten Forsterei in the German capital, even if the 41-year-old may have taken avoiding defeat prior to kickoff.
These two sides had already met twice in this season's competition having been paired together in Group D, with the away team winning 1-0 on both occasions. When Victor Boniface opened the scoring for the visitors last week, history looked set to repeat itself.
However, Union Berlin's first equaliser came via Josip Juranovic three minutes before the break, before the two sides exchanged blows twice more during a topsy-turvy second half, leaving everything to play for in Belgium on Thursday.
With his side closing the gap to five points behind Genk at the top of the Belgian Pro League in recent weeks, Geraerts will be supremely confident of leading the club to a historically successful campaign.
Union Berlin, on the other hand, have won just won of their last seven matches in all competitions, with five draws during that run halting their outside chance of winning Germany's top flight for the first time in the club's history.
Nonetheless, Fischer's side have enjoyed a remarkable rise since gaining promotion from the second tier in 2020, missing out on qualifying for the Champions League for the first ever time by a solitary point last season.
Like their forthcoming opponents and semi-namesakes, Die Eisernen's current run to the last 16 of the Europa League is already the furthest that the club has ever progressed in any European competition, but they will be determined to make it to the last eight in Saint-Gilles on Thursday.
Given that they remain in the DFB-Pokal ahead of facing Eintracht Frankfurt in the quarter-final next month and have still managed to hold onto a top-four position despite their frustrating run of form in recent weeks, Fischer's side have everything to play for between now and the end of the campaign, with their never-say-die attitude making them tough opponents for anyone.
Union SG Europa League form:
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W
D
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L
D
Union SG form (all competitions):
D
L
L
W
D
W
Union Berlin Europa League form:
W
W
W
D
W
D
Union Berlin form (all competitions):
D
W
L
D
D
D
Team News
Union SG appear to have a fully-fit squad at their disposal, with Christian Burgess' two-match suspension for an accumulation of yellow cards only applying to domestic fixtures.
After putting in a solid collective performance in Berlin last week, Geraerts may be tempted to pick the same starting XI, although Simon Adingra does provide a more attacking option on the left flank.
Union Berlin, meanwhile, are likely to travel without Rani Khedira, Andras Schafer and Danilho Doekhi due to the trio suffering from injury.
The former pair were substituted during their side's draw against Wolfsburg on Saturday due to muscle strains, while Schafer has already been ruled out until after the upcoming international break due to a foot problem.
Niko Giesselmann was also taken off against the Wolves due to illness and may not be risked, with Jerome Roussillon waiting in the wings to replace him from the start on Thursday. Timo Baumgartl and Janik Haberer could also earn recalls from their manager.
Union SG possible starting lineup:
Moris; Kandouss, Burgess, Van Der Heyden; Nieuwkoop, Amani, El Azzouzi, Lynen, Adingra; Vertessen, Boniface
Union Berlin possible starting lineup:
Ronnow; Baumgartl, Knoch, Leite; Juranovic, Thorsby, Laidouni, Haberer, Roussillon; Becker, Behrens
We say: Union SG 1-2 Union Berlin
With both clubs on the cusp of reaching the quarter-finals of European competition for the first time in their history, this is set to be a fiercely-contested second leg.