Mats Hummels admits there is no guarantee Borussia Dortmund will avoid relegation next May after last season's runners-up finished the first half of the Bundesliga season joint bottom on Sunday.

"There is no guarantee things will improve," the Dortmund captain admitted to German daily Bild after his side were beaten 2-1 at Werder Bremen on Saturday.

"Almost all of our first-choice players were missing at some point in the first half of the season and weren't always 100 percent fit.

"We should manage to get everyone fit during January's winter break, then we can show a different side when we have our best 15 or 16 players.

"We have had the worst half to the season anyone could have imagined. The worry has been considerable for weeks and the way we are playing, we are right down there."

Dortmund were last relegated from Germany's top flight in the 1971/72 season, when they finished the first half of the campaign in 15th compared to their current position of 17th.

Despite scoring Dortmund's consolation goal in Bremen, World Cup-winner Hummels was one of several below-par performers and was beaten for pace by Bremen's Davie Selke in the build-up to Werder's second goal.

The point bottom side Freiburg earned in Sunday's 2-2 draw with Hanover meant Dortmund finished the weekend joint last.

Both Dortmund and Freiburg have a goal difference of minus eight and their points tally of 15 is Borussia's worst at this stage since the 1987/88 season.

Dortmund qualified for the last 16 of the Champions League -- where they will face Juventus -- after winning their group ahead of Arsenal, but they have endured the worst start to a Bundesliga season for 27 years.

Having finished as runners-up to Bayern for the last two years, this is the first time Dortmund go into the winter break in a direct relegation spot in the club's history.

Their 10 defeats and just four points picked up away from home are both top-flight low marks.

"The fact that we look like complete idiots on the pitch, serves us right," said coach Jurgen Klopp.

"We played the shittiest first half of the season in our lives."

But Klopp has said he will ensure Dortmund put in a more convincing display when the season resumes at the end of January with a trip to another of Germany's Champions League representatives, Bayer Leverkusen.

"It will be much more difficult to beat us," added Klopp.

"We're not that far away from climbing out of trouble. We will be a fierce hunter (of those above us), I can promise that."