Union Berlin have a habit of stepping it up a notch against more illustrious opponents. Last season they were unbeaten in four games against the second division's two richest clubs, Cologne and Hamburg. They then went on to defeat Stuttgart across a two-legged playoff to secure promotion to the Bundesliga for the first time in their history.

But their giant-killing exploits came crashing down when they hosted RB Leipzig in their Bundesliga debut. The 4-0 spanking was kind as the debutants were ruthlessly dispatched by a rampant, energetic, and classy opposition.

Nerves crept in as the team made uncharacteristic defensive errors time and again in the first half. The organization that defined their promotion-winning campaign last season disappeared in the face of superior adversaries.

Read more: Union Berlin - a club between idealism and reality

In the third game of the season, Union Berlin welcome Borussia Dortmund - the league leaders, top goalscorers, and favorites for the title - to the capital. "Is there even an easy opponent for us in the Bundesliga?" Head coach Urs Fischer's words when asked about the difficulty of facing Dortmund after the 1-1 draw with Augsburg are a reminder of how huge a task avoiding relegation is for Union.

Glimmer of hope

While an improved display in last weekend's draw against Augsburg will have provided some hope, there will be few people tipping an upset at the Alte Försterei on Saturday.

Nevertheless, Union have come close to sending shockwaves through Dortmund in recent years.

In 2016, they traveled to the Westfalenstadion for the German Cup second round, with Steven Skrzybski scoring a late leveler to send the match to penalties. Union missed every single spot kick.

Last year, they returned to Dortmund at the same stage of the cup, and took the match to extra-time before falling to a last-minute Marco Reus penalty.

Of course, cup football is a different beast and Dortmund had rested some of their first-team regulars, but Union impressed with how they took the game to Dortmund, implementing a high line and a proactive approach.

Defensive security and the eradication of basic errors are likely to be the priorities, though. "It's difficult to defend against Dortmund's speed. But after the Leipzig game we have some ideas about what we should do better,” coach Urs Fischer said. "We must not repeat the same mistakes.”

Secret weapon

Enter Neven Subotic. A two-time Bundesliga winner with Borussia Dortmund, the 30-year-old defender admitted ahead of the match that "BVB is part of my being". His experience and inside knowledge could help Union, but the Serbian is still rediscovering his form and fitness after injury. A few shaky moments against Augsburg where he cheaply gave away possession and took up surprisingly poor positions does not bode well.

With Keven Schlotterbeck suspended and midfield lynchpin Grischa Prömel injured, an in-form Subotic is vital to any chances Union may harbor. "Defensively it will be an adventure for us," Subotic admitted. "With [Jadon] Sancho coming with his 200 kilometers per hour, [Marco] Reus running deep and Paco [Alcacer] also looking for a gap, we cannot afford such mistakes that we still had in Augsburg."

In attack, Union will have to rely on set pieces - still an issue for Dortmund - and their speed and composure on the break.

High-flying Dortmund

If anyone is going to succeed on the break though, it's Dortmund. Despite their lingering defensive fragility, Dortmund's pace in transition can hurt any side, let alone a team still finding their feet in the first division. Eight goals scored in just two matches from the likes of Alcacer, Reus and Sancho will leave Union with a headache - one made worse by the news that Julian Brandt is expected to replace the injured Thorgan Hazard after his game-changing performance off the bench against Cologne.

"I don't know who will play and what system we'll use," a cagey Lucien Favre said. "[Union] are very well organized, very athletic. That could cause us problems."

This game could not be more of a mismatch, and Dortmund will want to produce their own statement win after RB Leipzig's near flawless performance on opening day.

Union Berlin captain Christopher Trimmel is certainly under no illusions about his side's chances of an upset. "Even if they played their so-called reserves, it would still be a quality team," he said. "Either way, we're the underdog. If we have a very good day and BVB a bad one, then we can get something."

With extra time and penalties not a possibility, perhaps Union can make it three draws in 90 minutes against Dortmund. At least this time, they would have something to show for it.