The visitors, on the other hand, are having a splendid second season in the German Bundesliga and will be hoping to end their recent poor run of form to move back into European qualification contention.
Former manager Jan-Moritz Lichte was sacked by the Mainz hierarchy at the end of December after the 41-year-old had attained just six points in the league throughout 2020-21.
That awful run at the start of the current campaign has left the Opel Arena outfit in 17th position, seven points behind Arminia Bielefeld, who occupy the relegation playoff place.
Bo Svensson was the man chosen to replace Lichte, and the former Die Nullfunfer youth team coach has picked up four points in his first five games.
Although the results under their new Danish manager have left some room for improvement, the performances have generally improved. A 1-1 draw with Borussia Dortmund and a shock 3-2 victory over RB Leipzig have installed renewed hope of an unlikely escape from the drop.
The Mainz faithful will know it will take something special to retain their top-flight status for another season, but three points over the weekend would leave them with a very real chance of doing just that.
Union Berlin have won just one of their last five outings to fall to eighth in the top-flight standings.
Despite their recent struggles, they are only four points behind Eintracht Frankfurt, who are in the final Champions League qualification place, thanks to their terrific start to the season.
Experienced striker Max Kruse has been pivotal for Eiserne in 2020-21. The 32-year-old summer signing has rolled back the years, bagging six goals and providing five assists in just 10 appearances.
However, Kruse tore a muscle back in December, and Union have not had the same attacking impetus without him.
Urs Fischer has done an outstanding job at the Stadion An der Alten Forsterei throughout his time at the club, and another three points over the weekend would maintain their charge towards continental football in the next campaign.
Mainz 05 Bundesliga form: LLDLWL
Union Berlin Bundesliga form: WDWLLDD
Central-midfielder Pierre Kunde Malong is the only absentee for Mainz, with the 25-year-old unavailable after failing to recover from knee surgery in time for this one.
As previously mentioned, Kruse continues to suffer from a torn muscle and is ruled out of this weekend's clash.
Mainz 05 possible starting lineup:
Zentner; Niakhate, Bell, St. Juste; Mwene, Fernandes, Kohr, Da Costa; Onisiwo, Szalai, Quaison
Union Berlin possible starting lineup:
Luthe; Friedrich, Knoche, Schlotterbeck; Ryerson, Gentner, Andrich, Ingvartsen, Giesselmann; Awoniyi, Teuchert
We say: Mainz 05 1-2 Union Berlin
Both sides will fancy their chances coming into this one, but we think the visitors should have too much quality for their struggling hosts. Union Berlin are chancing European qualification this season and should continue their charge with another three points on Saturday.