Chelsea are joint leaders with Bremen on 10 points, two ahead of third-placed Barcelona while Levski remain last with no points.

Barca will host Bremen in the last group stage clash, which will be decisive in determining the second team who will book a place in the competition's second round along with group winners Chelsea.

At the Weserstadion, an unmarked Mertesacker scored his first ever Champions League goal in the 27th minute when he received Torsten Frings' corner with a downward header that he buried past Chelsea keeper Carlo Cudicini.

Chelsea almost leveled the score seven minutes later after Joe Cole sent a right-footed cross from the left-flank inside the area to Michael Ballack whose effort was parried by Bremen keeper Tim Wiese from close range.

Cole had another chance for the visitors three minutes before the interval when he rounded Naldo inside the area but his poor shot was easily collected by Wiese.

In the second-half, Chelsea laid siege on Bremen's area but failed to break the Germans' resilient defense.

The English champions suffered a blow just before the hour mark when Ivorian striker Didier Drogba was injured and replaced by Andrei Shevchenko.

Cole continued his threat in the 75th minute but another brilliant save from Wiese blocked his left-footed low drive.

Elsewhere, two goals from Ludovic Giuly and Andres Iniesta gave Barcelona a 2-0 away victory over Levski Sofia.

Levski's resistance didn't last for long as Barcelona grabbed the lead only five minutes into the game.

Gianluca Zambrotta raced down the right-flank before sending a low cross that deflected off a defender and went to the darting Giuly, who drilled a low shot through the keeper's legs.

Iniesta ensured the victory in the 65th minute as he scored from the rebound after Deco's initial shot was blocked by the keeper.