In a meeting between 16th and 15th, a draw would not do either side much good, but a win would be priceless given how tight it has become at the foot of the Bundesliga standings.
Match preview
Mainz come into this fixture only outside of the automatic relegation places on goal difference after extending their winless run to seven by drawing 1-1 with Wolfsburg on their return from the winter break.
Having still only won one game all season - 2-0 at home to RB Leipzig in early November - Mainz needed a second-half equaliser from Silvan Widmer to ensure they did not start 2024 with a defeat.
Failing to beat both Koln and Darmstadt below them during their current winless run has been costly, as it saw them squander the chance to move clear of the bottom two.
It leaves their 15-year stay in the top flight under threat, especially with their incapabilities of winning matches.
A feeble attack has been largely to blame for that, as only Koln have scored fewer than their 14 goals in 17 games so far.
Union Berlin are not the ideal opponents for Jan Siewert's side to face next either, despite their struggles, as they have won just one of the previous nine meetings.
The visitors also bring solid form into the fixture after a woeful start ultimately saw the reign of the legendary Urs Fischer come to an end, as Union went on their worst run of results as a professional club.
Nenad Bjelica has come in, and within four games, has already matched the amount of points they won in their opening 12 games of the season.
Beating relegation rivals Koln shortly before the winter break felt like a pivotal moment, as it moved Union out of the relegation zone for the first time in three months, and they returned by holding a strong Freiburg side to a 0-0 draw away from home last weekend.
While that ended a six-game losing run on the road in the Bundesliga, it was their ninth blank of the season in just 16 games - an unwanted league-high, despite playing a game fewer than the rest.
Bjelica will look to make strides in improving their away form, as they have won just one in 14, and that came against bottom-placed Darmstadt back in August.
Even more so because a win here would be hugely important for the visitors, as it would momentarily take them above Bochum, and potentially six points clear of trouble.
Mainz 05 Bundesliga form:
D
L
D
L
D
D
Mainz 05 form (all competitions):
L
D
L
D
W
D
Union Berlin Bundesliga form:
L
D
W
L
W
D
Union Berlin form (all competitions):
D
W
L
L
W
D
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Team News
Mainz will be without South Korean international Jae-sung Lee for the next few weeks, as he is on duty at the Asian Cup with his country.
Stefan Bell could return after missing the draw against Wolfsburg with a virus, but teenager Nelson Weiper remains out after knee surgery back in September.
Kevin Behrens scored a hat-trick when Union beat Mainz 4-1 on the opening day, but he lost his place in the side on their return to action.
With Sheraldo Becker injured, and David Datro Fofana departing the club, Mikkel Kaufmann made his first start for Union since joining from Copenhagen in the summer ahead of Behrens.
Union are missing two key midfielders for the foreseeable future though, with Rani Khedira injured, and Aissa Laidouni at the Africa Cup of Nations with Tunisia.
After Leonardo Bonucci left for Fenerbahce following a very unsuccessful half-season at the club, Kevin Vogt has arrived from Hoffenheim to fill the gap and he made his debut against Freiburg last week.
Mainz 05 possible starting lineup:
Zentner; Van den Berg, Kohr, Caci; Widmer, Papella, Kruass, Mwene; Fernandes, Onisiwo, Burkardt
Union Berlin possible starting lineup:
Ronnow; Knoche, Vogt, Diogo Leite; Juranovic, Kral, Aaronson, Gosens; Haberer, Kaufmann, Volland
We say: Mainz 05 0-1 Union Berlin
Goals have not been the strength of either side, but Bjelica has shored up the Union backline since coming in as manager, and they may just have the individual quality to snatch a win here.