Merkel arrived in Istanbul on Sunday evening needing to walk a tightrope balancing different political interests on her shoulders.
She spent months negotiating the refugee pact, which sees Turkey take back migrants who entered Europe illegally and in exchange send Syrian refugees from its own huge camps into Europe.
The deal, agreed by the EU with Turkey in March, will also see new parts of the negotiation for Turkey's entry into the club opened and the promise of immediate visa-free travel for Turkish citizens.
Since then, Erdogan has faced an avalanche of criticism in Germany, as he has filed legal proceedings against a German comedian, apparently forced out his more moderate prime minister, and pushed through a new law lifting MPs' immunity - a move seen as being aimed at the country's Kurdish minority.
On Sunday, Merkel met representatives of Turkish civil society, including journalists, human rights defenders and lawyers, but there were no opposition politicians among them.
With the situation so delicate, she fell back on her default posture of giving away very little about her upcoming meeting with Erdogan in an interview with the Frankfurter Allgemeine Sonntagszeitung (FAS).
While she acknowledged “great concern” at the removal of immunity from MPs – allowing for prosecutions of Kurdish lawmakers – she said only that she would address “all the important questions.”
And the Chancellor pushed back on suggestions that she had made herself dependent on Erdogan.
“Of course there are mutual dependencies,” she told the FAS. “You can also just call it the need for balancing our interests.”
Meanwhile, tabloid Bild reported on Monday that German officials think it's unlikely Turkey will meet all the requirements for visa-free travel before 2017.
Critics across the spectrum
At home in Germany, meanwhile, Merkel's visit has drawn a chorus of criticism from across the political spectrum.
“The end doesn't justify any means,” said Horst Seehofer, head of the Christian Social Union (CSU), Merkel's conservative Bavarian allies.
“The whole world should be shouting out” about the removal of Turkish MPs' immunity, the Bavarian minister-president went on in an interview with public broadcaster ARD.
Merkel's junior coalition partner, the Social Democratic Party (SPD), also voiced concerns.
“Germany cannot stay silent” about Erdogan's increasingly autocratic ways, the SPD leader in the Bundestag (German parliament) Thomas Oppermann told Bild am Sonntag.
“Merkel must not fold for Erdogan just because he keeps the refugees at bay for her and for Europe,” said Anton Hofreiter, leader of the opposition Green Party MPs.