Dortmund came into the match heavily favoured to win and reclaim their place on top of the table but struggled to match Berlin's enthusiasm in sweltering conditions in the German capital.

Union were 1-0 in the 22nd minute, with Marius Buelter scoring the side's first ever top-flight goal at home.

Buelter latched onto a Berlin corner which had been struck low and fast, hammering the ball into the bottom right of the net.

The goal stung Dortmund into action, who equalised just two minutes later through Paco Alcacer.

Dortmund coasted through the remainder of the first half, dominating possession and territory as they went in search of what seemed like an inevitable second.

It was Union who would score next, however, with Buelter grabbing a second by pouncing on a desperate clearance from Dortmund keeper Roman Buerki.

- Bayern dominate Mainz -                  
                  
Earlier on Saturday, Bayern Munich new boy Ivan Perisic bagged his first goal for the Bundesliga champions as they romped to a 6-1 win over a hapless Mainz at the Allianz Arena.

Robert Lewandowski scored his sixth of the season in just three games, while Benjamin Pavard, David Alaba, Kingsley Coman and Alphonso Davies also found the net.

In Freiburg, Cologne's Ellyes Skhiri scored the winner in second-half injury time to secure the visitors a 2-1 victory.

Freiburg took the lead in the 40th minute through an own goal by Rafael Czichos, before Cologne fan favourite Anthony Modeste equalised in the 52nd minute.

Schalke picked up their first win of the season over a disappointing Hertha Berlin at Gelsenkirchen's Veltins Arena. 

Hertha scored own goals either side of half-time through Niklas Stark and Karim Rekik.

English defender Jonjoe Kenny added a third in the 86th minute to make it 3-0 -- the first goal by a Schalke player this season -- to put the game beyond doubt.

Wolfsburg and Paderborn fought out a 1-1 draw at the Volkswagen Arena.

Newly-promoted Paderborn went ahead through Cauly Souza in the 12th minute, before Josip Brekalo equalised in the 56th minute.

At the BayArena, visitors Hoffenheim ended Bayer Leverkusen's 100 percent start to the season as the sides fought out a 0-0 draw.
The visitors had to endure a six-minute spell of injury time but held firm against a luckless Leverkusen attack.

Sebastian Andersson added another in the 75th minute to put the home side 3-1 up and seal their first ever win in the top division.

Union manager Urs Fischer said he was proud of his side's performance -- and planned on letting them savour the victory.

"We knew that it would be a really difficult 90 minutes -- when you want to take points off (Dortmund) you have to work hard and try everything. And we did it," Fischer said.

"I think there might be a beer or two tonight -- we'll see if we get around to completing the video analysis tonight or not."

Dortmund captain Marco Reus said the entire team was to blame for the loss.

"We had plenty of opportunities but the last pass seemed to fail us every time," he said. 

"We simply didn't put in enough tonight."