Union will remain top no matter how many goals Bayern scores against Hoffenheim next weekend, meaning the team from the capital will be leading Germany's top soccer division for at least six consecutive weekends.
It's a remarkable achievement for a club that was only promoted to the Bundesliga for the first time in 2019 and has seen its top performers leave in every transfer window for better-paying rivals.
Union's only target for its fourth season in the topflight was to avoid relegation.
But the Kopenick-based club is exceeding even its own expectations. Janik Haberer struck twice for a 2-0 win over Borussia Dortmund on Sunday, when the home team might also have gone on to score more.
Union's players celebrated with the fans afterward, jumping up and down on the field as the supporters sang about winning the league title.
It was Union's seventh victory in 10 league games. It also drew with Bayern 1-1 last month.
``It's crazy, actually unbelievable, too,'' said Union coach Urs Fischer, who dismissed any notion of a title challenge. Fischer said his team's target remains 40 points and staying up ``before maybe talking about a new target.''
At the same stage last season, Bayern was leading by a point ahead of Dortmund. It was a point ahead of Bayer Leverkusen after 10 games the season before.
Union is the only team playing in the Bundesliga to have played in the DDR-Oberliga - the old East German league - and its fans have to stretch their memories back to the first matchday of the 1970-71 season to remember when their team was last top of the table. The only other time Union was first in the Oberliga was after the seventh round of the 1967-68 season.
``Of course, I understand it,'' Fischer said of the excitement being generated by this team, ``but I don't care. It's logical that that our fans celebrate us, that they throw themselves into a campaign for the championship, all good and correct. They can dream. But we're not the ones who should dream, but who should retain focus.''
Much of Union's success is down to the level-headed Swiss coach. Fischer took over in 2018 and led the team to promotion via a playoff in his first season. Union finished 11th after its first year in the Bundesliga, then seventh, then fifth last season to qualify for the Europa League.
Union's intensive first-half performance against Dortmund encapsulated everything Fischer wants from his players. The visitors had no time on the ball with Union's players challenging at every opportunity, winning most, while every 50-50 ball seemed to go the home team's way.
Dortmund coach Edin Terzic was effusive in his praise for Fischer's team afterward.
``It's what sets top teams apart. Everyone knows what they can do but they can't stop it. If you look over the past few years, it's a very mature, very, very ingrained style of play. You can see that everyone knows their job. They're not above doing things that are important,'' Terzic said what seemed like a thinly disguised barb against his own players.
``There are a couple of things they're happy to concede, that's maybe ball possession and risk building up the game. But what they don't avoid are challenges. They throw themselves into every challenge and are very, very aggressive. They help each other, support each other, never leave a teammate alone, but always come back, so it's not easy to play your way through because you're generally hit by two or three challenges within a few seconds.''
Union has the best defense in the league, but Fischer says there's still much room for improvement.
``We still have our issues that we need to work on,`` said Fischer, who referred to the team having just 29% ball possession and a successful pass rate of 68%.
``It has to be more precise,'' he said.
Union hosts second-division club Heidenheim in the second round of the German Cup on Wednesday before it visits last-place Bochum in the Bundesliga on Sunday.
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