But the referee ruled two of his teammates were offsides, although the two players did not figure in the play.
"We played fantastic the first 20 minutes, but failed to score a goal," Berlin coach Falko Goetz said. "After that both teams just hung back and waited for the other to make a mistake."
Bremen kept alive its flickering hopes of retaining the title while Rostock was buried deeper in the cellar. The East German team is nine points away from 15th place, which would keep it in the league.
But Bremen was also lucky when Germany defender Frank Baumann wasn't whistled for a hand ball in the area.
"From the sideline I saw a clear penalty -- it's a mystery why that wasn't called. It would have been a very important point for us," Rostock coach Joerg Berger said.
Klasnic scored in the fifth and 62nd minutes and teammate Miroslav Klose in the 32nd, while Rostock's goals came from David Rasmussen in the 45th and Thomas Rasmussen in the 64th.
Serie A leader Juventus beat last-place Atalanta 2-1 to open up an eight-point lead over defending champion AC Milan, which slumped to a 1-0 home loss against lowly Bologna.
Juventus won at Bergamo on a first-half goal by Uruguayan midfielder Ruben Olivera and a second-half penalty converted by star forward Alessandro Del Piero, fielded minutes earlier as a second-half substitute. Olivera scored in the 23rd minute from outside the area with a lob which caught Atalanta goalkeeper Alex Calderoni out of position.
Del Piero converted the penalty in the 80th for his seventh goal this season after Mauro Camoranesi was fouled deep in Atalanta area.
Braga defeated defending European champion FC Porto 3-1 to move into first place in the league.
Striker Joao Tomas scored on a header for Braga in the 21st minute, but Porto's Diego Ribas da Cunha equalized in the 33rd. Midfielder Wenderson Wender scored on a header in the last minute of the first half and Thomas sealed the win with a goal in the 75th.
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