Borussia Dortmund boss Hans-Joachim Watzke has repeated his job guarantee for under-fire coach Jurgen Klopp as last season's runners-up battle to escape the Bundesliga's relegation places.

The 47-year-old Klopp has a contract until June 2018, but Dortmund's dire form has led to questions about how much longer he can stay in the job, even his team has finished in the Bundesliga's top two for the last four seasons.

Watzke has always said Klopp will only leave Dortmund when he chooses too and Borussia's chief executive officer insists the coach still has the confidence of his superiors.

"Of course. There is no question about that," Watzke told Bild, adding that he is not prepared to answer questions about Klopp's future at the club on a weekly basis.

The German daily also reported on Friday that ex-German league coaches Armin Veh and Thomas Tuchel are two candidates poised to step in should Klopp leave the club.

Dortmund climbed off the bottom of the league table on Saturday with a 3-0 win at fellow strugglers Freiburg to claim their first victory since the start of December.

And Klopp has said he is convinced Dortmund will stay up this season.

"I am absolutely convinced that our mission will succeed -- so accordingly it's not a subject for me," he said last Thursday when asked if he was considering resigning.

Dortmund face the litmus test of their season in the next fortnight.

They host mid-table Mainz on Friday, then fellow strugglers Stuttgart away on February 20 before their Champions League last 16, first-leg clash at Juventus.

Then comes the Ruhr Valley derby at home to Schalke four days later.

"The next three weeks will decide which way it will really go," said Bayern Munich striker Robert Lewandowski, who added that he "can't imagine" his former club in Germany's second tier.