Jens Keller, fresh from their hard-fought nil-all draw against Dynamo Dresden last weekend, was forced into one change. Toni Leistner was suspended, so Emmanuel Pogatetz came in as his replacement for Union's only alteration.
As for Bielefeld, Jürgen Kramny's side were made to go the distance against FC-Astoria Walldorf as they won 5-4 on penalties. Despite the huge exertion of energy to earn a a win, he opted for a similar side. Julian Börner, Christoph Hemlein and David Ulm made way for Brian Behrendt, Reinhold Yabo and Andreas Voglsammer, however.
The hosts started quickly and with purpose, Felix Kroos' early corners caused real trouble in the early minutes. Kristian Pedersen had a half volley deflected narrowly over before a second ball from a poorly cleared set-piece saw Pogatetz's cut-back trundle across the face of goal.
Things almost went from bad to worse for the visitors as a mix-up between Sebastian Schuppan and Wolfgang Hesl saw the ball drop for Steven Skrzybski in the area. Fortunately for Kramny's men, Sebastian Schuppan and Hesl both put in great blocks and after a free-kick was given away on the edge of the area, the danger finally passed.
The pressure continued as Polter flicked over Skrzybski's cross but Arminia's luck ran out soon after. Michael Görlitz's foul looked like a reasonable decision, given it was 25 yards from goal, though Kroos stepped up and sent a superb curling effort up and over the wall to find the top corner.
That hard work was almost quickly undone when Pogatetz dithered on the ball and failed to clear. It dropped to Voglsammer who tried to play in Yabo, but the ball was just too much for the on-loan midfielder to control and the chance passed.
Bielefeld would take their only major chance just before the break and Klos' grabbed his ninth goal of the season in doing so. Michael Görlitz's cross to the near post was met well by Voglsammer, with his effort catching Jakob Busk off-guard and forcing a good save from the 'keeper. However, captain Klos was quick to react and slid in the rebound.
Kreilach and Polter grab all three points
The second period began with both sides pushing forward and made for a very lively first 15 minutes. There were chances at either end as Voglsammer's stinging effort brought a good reaction stop from Busk, while Polter continued to force the visiting defence backwards with his superb hold up play.
However there would be a breakthrough not long after as Union retook the lead. It was a fine move that eventually allowed Christopher Trimmel to drive down the right wing. The Austrian clipped a cross to the back post, where it evaded the attentions of everyone and caught out Hesl to leave Kreilach free to nod home.
The game continued to flow back and forward after that, but the hosts continued to come up short in terms of finding the crucial goal to put the game beyond doubt. They could very easily have been punished for their profligacy, though Busk's flying save from Schuppan's volley prevented Bielefeld from levelling.
At the death, Union managed to get their third goal and seal all three points. A corner was whipped into the near post before taking two deflections that landed the ball straight with Polter. He prodded home to move the Berliners into fourth, level on points with Hannover 96. As for Bielefeld, they slipped into the bottom two thanks to the loss.