"It's a big compliment for the SPD if Angela Merkel is longing to be back with the FDP [Free Democratic Party]," he told Bild.
"Of course it's more difficult with us than with the FDP. Governing properly is always tougher than letting things coast.
"I don't begrudge her and the CDU [Christian Democratic Union] their short break at the party conference carnival in Cologne. But starting on Thursday, the Cabinet table and hard work are waiting in Berlin."
The Chancellor said that her party would look to the Greens or the FDP as coalition partners following elections in 2017.
But SPD vice-president Ralf Stegner argued to Bild that it was the CDU who were short of political ideas.
"If Mrs Merkel attacks the SPD fiercely in her party conference speech and mourns the losing black-yellow [CDU-FDP] coalition that was voted out, that just shows how successfully the SPD is dominating government policy with the minimum wage, pensions and gender equality," he said.
"The Union's greatest hits, road tolls for foreigners and subsidies for keeping children out of kindergarten, are damaged goods by comparison."