BREMEN, Germany, Nov 22 - Chelsea squeezed through to the last 16 of the Champions League on Wednesday despite suffering a 1-0 defeat to Werder Bremen, who are now just a point away from pulling off a great upset in Group A.

Per Mertesacker, the tall German defender, took advantage of poor marking at a corner to power in a header in the 27th minute.

Michael Ballack, the man who failed to pick up Mertesacker, should have made amends but the Germany captain made a mess of the best Chelsea chance of the game, giving Tim Wiese an easy save when he met Joe Cole's perfect cross only with his thigh.

Chelsea stay top of the group, level on 10 points with Werder but ahead of them on the head-to-head record thanks to their 2-0 win in London.

Barcelona, who are third with eight points, now need to beat Werder at the Nou Camp in a fortnight's time to overtake the Germans and avoid becoming the first title holders to fail to go through from the group stage.

"We set ourselves high aims in this group and so far we've reached all of them," Werder coach Thomas Schaaf told reporters. "Now we just need to put the icing on the cake."

Barca have plenty to worry about judging by the combination of technique and temperament shown by Werder on Wednesday.

Schaaf's side made their customary high-tempo start, with both full backs testing the Chelsea defence with their breaks forward.

Pierre Wome's hanging cross from the left presented Miroslav Klose with a great chance in the seventh minute but the Germany striker was not quite able to connect.

Hugo Almeida sent a skidding shot wide after Diego's close control in midfield had set him up in the 10th minute.

INFLUENTIAL DIEGO

Diego then brought a solid save from Carlo Cudicini with a sweetly timed effort from the edge of the box and the Brazilian midfielder, easily the most influential player on the pitch, set up Almeida for another good chance in the 17th minute.

Chelsea could feel aggrieved about the Werder goal, which came after Torsten Frings had won a free kick by falling to the ground.

Mertesacker met Frings's corner with a weak header that Didier Drogba was able to clear but from the resulting corner the German international was left unmarked by Ballack to head down and in.

Ballack's poor return to Germany continued with his weak effort from Cole's wonderful cross, struck with the outside of his foot, in the 33rd minute.

Werder spent most of the second half in increasingly desperate defence and they needed two fine saves from Wiese to safeguard their win.

The first, at full stretch, came from a cracking free kick from substitute Andriy Shevchenko in the 70th minute low to the keeper's left.

The second was just as good, with the keeper again diving low, this time to his right, to keep out Cole's low drive.

Drogba limped off midway through the second half and captain John Terry will miss Chelsea's final group game at home against Levski after being booked.

"We dominated for the last 20 minutes but we had no power in the box," said Chelsea coach Jose Mourinho. "I think maybe a draw would have been a fair result but I accept Bremen fought very well to earn these three points."