Sigmar Gabriel, Germany's unexpected foreign minister

PIC: German President Joachim Gauck (L) hands over the certificate of appointment as German Foreign Minister to Sigmar Gabriel after his formal resignation as Economy and Energy Minister on January 27, 2017 during a dismissal and appointment ceremony at the presidential Bellevue Palace in Berlin. (AFP)

Germany's outspoken vice-chancellor Sigmar Gabriel, better known for his stormy temperament than his diplomatic finesse, is the first to admit he's an unlikely choice for foreign minister.

But that's exactly the role the burly 57-year-old finds himself in on Friday after abandoning his long-held ambition to challenge Chancellor Angela Merkel for the top job, bowing to his low approval ratings.

In a head-spinning round of political musical chairs this week, Gabriel said European Parliament head Martin Schulz would replace him as leader of the Social Democratic Party (SPD) and take on the mighty Merkel in September's general election.

In the same breath, he announced he would step down as economy minister to take over the foreign ministry portfolio from Frank-Walter Steinmeier, who is expected to become the country's next president in February.

For a man who once compared Israeli policy towards Palestinians to an "apartheid regime", Gabriel's new role as Germany's top diplomat has raised eyebrows.

The conservative daily Die Welt called the move "a joke" in an editorial and said it feared Gabriel would crash onto the international stage "like a bull in a china shop".

Gabriel acknowledged the criticism in his final press conference as economy minister, but added he was confident he could learn to speak "diplomacy German".

"I like to think I'll be able to refrain from triggering any foreign policy crises with my words," he quipped.

Baptism of fire

Faced with the unpredictability of a Donald Trump presidency in the United States and the start of Britain's divorce talks from the European Union, Gabriel can expect a diplomatic baptism of fire.

His first test will come as early as next month when Germany hosts a G20 gathering of foreign ministers, also expected to see the international debut of Trump's pick for US secretary of state, Rex Tillerson.

Gabriel, who labelled Trump's inauguration speech "highly nationalistic", has warned that the world should brace for a rough ride under the US billionaire.

But he has also urged Germans to face the coming geopolitical challenges "with confidence", and said any US withdrawal from the world stage should be seen as an opportunity for Europe to forge closer ties with countries like China.

As the new face of Germany's foreign policy, Gabriel will be hoping to put past gaffes behind him.

In 2015, he became the first top Western official to visit Iran following its historic nuclear deal. But the trip -- already viewed with scepticism in Berlin -- descended into controversy after Gabriel said cooperation between the two countries would require Iran to recognise Israel.

He also ruffled feathers in Beijing when he accused the Asian giant of "unfair" trade practices ahead of a visit there last November, leading to a chilly reception from his Chinese hosts.

Staunch Nazi father

A former teacher who took the reins of the SPD in 2009, Gabriel has seen his popularity tumble since he joined Merkel's coalition cabinet in 2013 as deputy chancellor and economy minister.

The father-of-two started out with high hopes of shifting government policy to the left, but critics say he has failed to make his party's achievements stand out while forever governing in Merkel's shadow.

Support for the SPD has eroded on his watch, its voters flocking to the far left and even the rightwing populist AfD party on concerns over immigration.

The SPD is currently polling at around 20 percent, trailing Merkel's CDU/CSU block with some 37 percent support.

Born in the northern town of Goslar, Gabriel had a complicated childhood. He revealed in a 2012 book that he had a difficult relationship with his father, who remained a staunch Nazi until the day he died.

When asked by Die Zeit newspaper how his father's past had affected him, Gabriel said it left him with an "almost uncontrollable anger".

"When I perceive something as unjust, when people are being wronged, I can get really worked up."

He caused a stir last year when he flipped his middle finger at a group of rightwing hecklers who were shouting praise for his father's beliefs.

"My only mistake was not using both hands," he said afterwards about the decidedly undiplomatic gesture.