With a trip to Real Madrid to come on Wednesday, Union first must overcome another club with top-four ambitions this season.
Match preview
After an underwhelming campaign last season, Wolfsburg are determined to get back into Europe after two successive failures.
Their wealthy owners have spent well, adding plenty of promising players all over the pitch, and it has paid off with a strong start.
Two wins from their opening two games has set the benchmark, and it represents a huge improvement from last season, when it took them eight games to reach six points.
Jonas Wind's early form has been vital as the Danish striker netted back-to-back braces in their two opening wins.
The 24-year-old also provided the assist for the opening goal at Hoffenheim before the international break, but Niko Kovac's men ultimately fell to a 3-1 defeat.
Being back on home soil after two straight away games on paper looks like a positive, but Wolfsburg must improve on their form here.
A win over Heidenheim on the opening day was a step in the right direction, but Wolfsburg only won two out of nine at home in the second half of last season.
The next three fixtures will provide a huge test of Kovac's new-look squad, with Union Berlin up next, before equally challenging tests against other European contestants Borussia Dortmund and Freiburg.
For Urs Fischer and Union Berlin, their biggest challenge will be maintaining focus and professionalism to ensure the trip to Madrid does not become a distraction here.
Like Wolfsburg, Union also started with two wins, but lost before the international break.
Mainz and Darmstadt were both thrashed 4-1 by a rampant Union, but RB Leipzig gave Die Eisernen a taste of their own medicine by winning 3-0 at der Alten Forsterei last time out.
Wind has four goals for Wolfsburg so far, and he is joined on that tally by Kevin Behrens, who scored a hat-trick against Mainz and bagged the third goal at Darmstadt.
Despite finishing fourth, Union kept the likes of Behrens and the rest of their strongest XI, while also adding plenty of strength ahead of their maiden Champions League journey.
Leonardo Bonucci has been the big-name signing, but adding Diogo Leite permanently from Porto, as well as Robin Gosens, Alex Kral, David Datro Fofana and Kevin Volland, despite his red card against Leipzig, makes them a stronger force.
Wolfsburg Bundesliga form:
Wolfsburg form (all competitions):
Union Berlin Bundesliga form:
Union Berlin form (all competitions):
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Team News
An Achilles issue picked up on Denmark duty by Wind means Wolfsburg's top scorer is a doubt ahead of this clash.
Kevin Wimmer will also undergo a late fitness test after suffering from illness and muscular issues during the international break.
Lukas Nmecha is absent after getting injured against Makkabi Berlin in the cup last month, with Kilian Fischer joining him on the sidelines.
With the Real Madrid game to come for Union Berlin, manager Fischer may be tempted into some squad rotation for this trip to the Volkswagen Arena.
The most obvious areas for rotation are at wing-back where Union have plenty of strength in depth, so Christopher Trimmel and Jerome Roussillon may start instead of first-choice duo Josip Juranovic and Gosens.
Brendan Aaronson will return from suspension after his sending off against Darmstadt and he should replace the man who deputised for him, Volland, as he will now serve a three-match ban following his dismissal against Leipzig.
Sheraldo Becker and Lucas Tousart remain doubtful as they make returns from injury, but Laurenz Dehl will be out until November with an Achilles injury.
Midfield pair Andras Schafer and Rani Khedira are not far away from making comebacks, but this fixture may come too soon for them.
Wolfsburg possible starting lineup:
Casteels; Maehle, Lacroix, Zesiger, Rogerio; Svanberg, Arnold, Majer; Cerny, Tiago Tomas, Wimmer
Union Berlin possible starting lineup:
Ronnow; Doekhi, Knoche, Diogo Leite; Trimmel, Kral, Laidouni, Aaronson, Roussillon; Fofana, Behrens
We say: Wolfsburg 2-2 Union Berlin
Union took four points off Wolfsburg last season and knocked them out of the DFB-Pokal, but there is a different look about both sides this season, and it could be a close encounter.