(This November 21st story corrects to remove reference to Fillon voting against the EU Constitution in paragraph 13. He voted for it.)

French Prime Minister Francois Fillon and German Chancellor Angela Merkel address a news conference following their bilateral talks at the chancellery in Berlin, March 10, 2010. REUTERS/Fabrizio Bensch/File Photo

BERLIN (Reuters) - In 2007, on his first visit to Berlin as French prime minister, Francois Fillon gave Angela Merkel an antique edition of “Radioactivity”, a work by one of her heroes, fellow physicist Marie Curie.

The book, now prominently displayed in the Chancellery in Berlin, is a symbol of Franco-German friendship and a reminder that Fillon, favorite to win the French presidency after his resounding victory in the first round of the Republican primary on Sunday, is a known quantity in Berlin.

Particularly on the issue of economic reform, where Fillon is proposing a shock-and-awe approach involving deep cuts to public spending, he is viewed by Merkel’s government as an ally.

But on other issues, from Russia and Turkey to migration and Europe, Fillon could prove a far more difficult partner for Merkel, who announced on Sunday, as Fillon was surging to victory, that she would seek a fourth term as German chancellor.

Germany and France have been the driving forces of European integration for over half a century. And it is likely to fall to Berlin and Paris to try to lead the EU out of an existential crisis triggered by Britain’s decision to leave the bloc.

For months, people in Merkel’s entourage have been whispering that Alain Juppe, another former prime minister who came in a distant second to Fillon on Sunday, represented the best hope for achieving that.

Like Merkel, Juppe is staunchly pro-European. He is a voice of moderation on issues of immigration and national identity. And he supports a hard line on Russia that mirrors the stance that Berlin has taken.

But after Sunday, Juppe - who faces off against Fillon in a run-off vote one week from now - appears to be a long-shot and Germany must begin preparing for a French president who is likely to distance Paris from Berlin on a range of important issues.

NO EASY PARTNER

“The first reaction in Berlin is one of relief. Fillon stands for the kind of economic reforms that Germany wants to see,” said Claire Demesmay of the German Council on Foreign Relations (DGAP) in Berlin. “But if you look a bit deeper, it’s clear that he would not be an easy partner.”

Fillon, 62, and born four months before Merkel in the race-car city of Le Mans, developed a close relationship with Vladimir Putin when both were prime ministers from 2008 to 2012 and raised eyebrows by greeting the Russian president as “dear Vladimir” in a speech several years ago.

He has criticized President Francois Hollande for aligning France too closely with the United States and Germany against Moscow, and like U.S. president-elect Donald Trump, wants closer cooperation with Putin to resolve the conflict in Syria.

On migration, Fillon could end up closer to right-wing leaders in Hungary and Poland than Berlin. He has accused Merkel of underestimating the risks from Islamic militants with her decision to allow hundreds of thousands of migrants to enter Germany last year. And he is a critic of the EU-Turkey deal on migrants that was driven by Merkel.

Fillon voted against the Maastricht treaty, which laid the groundwork for the creation of the euro.

UNPREDICTABILITY

There was relief in Berlin on Monday at the exit of former president Nicolas Sarkozy from the French race.

From 2007 to 2012, Merkel and Sarkozy worked extremely closely together on Europe’s response to the global financial and euro zone crises, earning the moniker “Merkozy”.

But the relationship was always rocky and German officials had been leery about Sarkozy’s candidacy for two reasons: his adoption of what Berlin viewed as dangerously populist positions on immigration and the fear that he might struggle to beat Marine Le Pen, leader of the far-right National Front, if he faced her in a presidential run-off.

That risk is reduced with Fillon. A BVA poll in September showed him beating Le Pen in a theoretical runoff by 61 percent to 39 percent.

Still, some German officials voiced concerns on Monday that Fillon’s economic proposals - which include plans to cut 500,000 public employees in five years - could leave him vulnerable as a crowded French campaign unfolds, with centrist candidates like Emmanuel Macron and Francois Bayrou jockeying for position.

“Personally I have always favored Juppe because he had the best chance of rallying French voters of the right and left against Marine Le Pen,” a senior German diplomat said.

Demesmay of the DGAP added: “If Fillon does emerge as the candidate, then the (political) center is wide open ... Things become a bit less predictable.”

After the shocks of Brexit and Trump, it may be this degree of unpredictability that Fillon brings to the race that - like the subject of the book he offered Merkel back in 2007 - could prove truly “radioactive”.