With Bayern Munich not playing until Sunday, Union have the chance to go top of the table with a win here.
Match preview
Some scintillating form post-World Cup has seen Union Berlin win three games out of three in the Bundesliga and reach the quarter-finals of the DFB-Pokal.
It was much needed too as Urs Fischer's side took just four points from their final five games before the break, conceding 14 goals and losing to strugglers Bochum.
When it seemed their top four dreams would fade into the distance, they have been resolute to regroup during the intermission and come back stronger than ever.
Despite the stuttering form in October and November, they are still 14 games unbeaten at home, a run which stretches back to last February, when, strangely enough, they beat upcoming opponents Mainz.
The question now is whether they can maintain this form while fighting on three fronts, as the schedule will become pretty daunting starting from this month.
With top four, and even potentially an incredible title challenge on the cards in the Bundesliga, a quarter-final in the Pokal, and a preliminary round tie with Ajax in the Europa League all to come, there is a lot to look forward to for the club from the capital.
Mainz will be happy to have ended a six-game winless streak by beating Bochum last weekend.
Their last win before that was a 5-0 success over Koln, and they netted five again as a Karim Onisiwo hat-trick inspired them to a 5-2 win.
Languishing in mid-table, there is no real imminent threat of relegation for Mainz, but there is also no realistic chance of European qualification either.
This is not a happy hunting ground for Bo Svensson's side either as they have lost on their last two visits by an aggregate score of 7-1.
It will also be interesting to see how the side reacts after being thumped by Bayern Munich in the cup in midweek and whether it has dented confidence again after the German champions comfortably swept them aside 4-0.
Union Berlin Bundesliga form:
L
D
L
W
W
W
Union Berlin form (all competitions):
D
L
W
W
W
W
Mainz 05 Bundesliga form:
L
L
D
D
L
W
Mainz 05 form (all competitions):
L
D
D
L
W
L
Team News
Morten Thorsby is Union Berlin's only injury absentee as he is currently out with an abdominal strain.
The headline-grabbing signing of free agent Isco fell through at the last minute, but it was overall another very good, efficient window for Union.
Julian Ryerson left for Borussia Dortmund, but they added two full-backs with plenty of experience to fill that gap in the form of World Cup bronze medallist Josip Juranovic and Jerome Roussillon.
Tunisian international Aissa Laidouni could make his full debut for the club after arriving from Ferencvaros, as the midfielder made a 30-minute cameo versus Stuttgart last weekend.
Silvan Widmer went off injured for Mainz before half-time against Bayern so the right-back will be assessed before this one.
Goalkeeper Robin Zentner and attacker Jonathan Burkardt both remain out so deputy Finn Dahmen should start between the posts while new signing Ludovic Ajorque should retain his place up top.
Ajorque has arrived from Strasbourg where he has endured a difficult season so far, but he still scored an impressive 29 Ligue 1 goals in the last two-and-a-half seasons.
Alexander Hack was sent off in the cup but that suspension will not carry over into league matches.
Union Berlin possible starting lineup:
Ronnow; Doekhi, Knoche, Diogo Leite; Juranovic, Khedira, Roussilon, Seguin, Haberer; Becker, Behrens
Mainz 05 possible starting lineup:
Dahmen; Hanche-Olsen, Bell, Fernandes; Da Costa, Barreiro Martins, Kohr, Caci; Lee Jae-sung, Onisiwio, Ajorque
We say: Union Berlin 2-1 Mainz 05
The Union Berlin faithful will be hoping this fairytale lasts forever, and they will be pinching themselves if they can return to the top of the tree with a win here.