"My mood is very tense, like it would be just before a very difficult exam, but we are in better shape than we were at the start of the season," insisted Klopp on Thursday.

"Leverkusen is a very important game, one of 17 very important games now for us, but I have been looking forward to this since the final whistle of our last league match at Bremen."

Just over 18 months after losing the 2013 Champions League final to arch-rivals Bayern Munich, Dortmund are second from bottom and threatened with relegation from Germany's top tier football league, the Bundesliga.

Their domestic fall from grace has been even more dramatic on the back of a successful European campaign that has seen them reach the Champions League's last 16.

Like most teams, Dortmund had expected to drop a league point here or there after five of the squad returned weary from helping Germany lift a fourth World Cup title last July.

But Klopp's dejected figure on the sidelines, struggling to accept any one of Dortmund's 10 league defeats, became an all-too common sight. 

Having won the 2011 and 2012 German league titles, then finished runners-up to Bayern in 2013 and 2014, Dortmund suddenly found themselves at the wrong end of the table this term.

Defeats at Mainz, Schalke and Cologne preceded humiliating loses at Hertha Berlin and Werder Bremen in December.

"To finish the first half of the season in 17th was like going on holiday where you sleep on a bed of nails," Klopp told German daily Die Welt. 

Injuries to attacking midfielders Marco Reus and Henrikh Mkhitaryan, especially after Poland striker Robert Lewandowski had left to join Bayern, meant the goals dried up.

Klopp had to grapple with a growing injury list and loss of form from key players, but the 47-year-old admits there were pre-season hints to Dortmund's demise.

"It was relatively clear that it would be difficult," he said.

"Even during pre-season training we thought: it will be tight to be ready in time.

"We picked up six points from our first three games, which was fine.

"We went to Mainz in late September with the chance of going top, but we ended up losing.

"Then it was quickly down to blow after blow and due to more injuries, we could hardly react or have enough time to train and those players we had fit were soon overloaded.

"But at the same time, you're Borussia Dortmund, you are the runners-up, former champions, former Champions League finalists. 

"Wherever we went, people said "you'll turn it round", but on the pitch things turned into a riot in a sporting sense.

"It was a downward spiral."

Klopp is the first to admit he made mistakes, for example using players who insisted they were fit to help the cause, when their performances in training suggested otherwise.

CEO Hans-Joachim Watzke says their rivals have used Dortmund's plight to pick up points this season while Bayern run away with the title having built up an 11-point lead.

"We've seen many teams, who normally pick up points against Bayern Munich, have simply rolled over against them," Watzke told Munich-based newspaper Süddeutsche Zeitung.

"Instead, those teams (pick up points) against us.

"We are now the platform for the others to create something special."

Dortmund's goal in recent years has been to challenge Bayern and secure a Champions League berth for the following season, but this campaign is now all about avoiding relegation.

That would mean potentially losing stars like Reus, who can leave in June due to a clause in his contract, and Hummels, who has been strongly linked to Manchester United.

There is also the financial impact of a season without Champions League football, which would cost Dortmund around €20 million in lost revenue after tax.

"To miss out on €20 million would be tough, but would not bring us into financial difficulties," insisted Watzke, after the club only narrowly escaped bankruptcy in 2005.