In addition, the European Stability Mechanism (ESM), the euro zone's bailout fund, is an "effective" rescue mechanism and was now available, another source added. Major banks would be protected by the banking union.
The German government in Berlin could not be reached for comment.
It is still unclear how a euro zone member country could leave the euro and still remain in the European Union, but Der Spiegel quoted a "high-ranking currency expert" as saying that "resourceful lawyers" would be able to clarify.
Read MoreGreece unlikely to leave EU, but not impossible: Pro
According to the report, the German government considers a Greece exit almost unavoidable if the leftwing Syriza opposition party led by Alexis Tsipras wins an election set for Jan. 25.
The Greek election was called after lawmakers failed to elect a president last month. It pits Prime Minister Antonis Samaras' conservative New Democracy party, which imposed unpopular budget cuts under Greece's bailout deal, against Tsipras' Syriza, who want to cancel austerity measures and a chunk of Greek debt.
Opinion polls show Syriza is holding a lead over New Democracy, although its margin has narrowed to about three percentage points in the run-up to the vote.
German Finance Minister Schaeuble has already warned Greece against straying from a path of economic reform, saying any new government would be held to the pledges made by the current Samaras government.