Wolfsburg coach Lorenz-Gunther Kostner fears fatigue could harm his team as they go into the clash with Hertha Berlin.

After a tiring trip to Russia 10 days ago, Wolfsburg needed 120 minutes to see off Rubin Kazan and book their place in the quarter-finals of the Europa League on Thursday and Kostner fears the trials and tribulations of the last fortnight are beginning to show.

"I noticed even before the Kazan game that there was a certain degree of tiredness present," he said.

"It was both muscular as well as mental, but the team wanted to take the chance of reaching the quarter-finals of a European competition for the first time ever and of course that motivates you."

Hosting a side who have won just three games and are staring relegation in the face is another prospect, though.

"We had to play 120 minutes and that takes a lot out of you," added Kostner.

"And I don't know whether - against a team who are down in the dumps - we can lift ourselves up again."

At least the likes of Makoto Hasebe and Obafemi Martins, who played only from half-time on Thursday, should provide some freshness while Grafite, who missed Thursday's game due to injury, could also come into the team tomorrow.

But, still carrying a groin strain, he would not necessarily be a fresh pair of legs.

"We are being cautiously optimistic," said Kostner, who hopes to have last season's Bundesliga top scorer available for the clash at the VW-Arena.

Opponents Bremen will need to find every last ounce of energy if they are to revive their dwindling hopes of avoiding relegation this season.

A 2-1 defeat at home to Nurnberg last weekend saw them drop nine points adrift of safety with eight games of the season remaining.

"Despite the critical situation, we are still concentrating," said coach Friedhelm Funkel.

"We trained very hard on Tuesday and Wednesday and will be optimally prepared for the game with Wolfsburg."

A police presence ensured Berlin's training sessions were able to go ahead without any disturbances after fans raced onto the pitch in a threatening manner at the end of last weekend's defeat at the Olympia Stadion, forcing the players to seek refuge in the changing room.