US President Donald Trump is to meet his Russian counterpart, Vladimir Putin, for the first time at a G20 summit in Germany.
They have both said they want to repair ties damaged by the crises in Syria and Ukraine, as well as Russia's alleged meddling in the US election.
Climate change and international trade are set to dominate the summit, which is drawing mass protests.
Clashes at a "Welcome to Hell" rally left 76 police officers injured.
What can we expect from the Trump-Putin talks?
The two leaders are due to meet in the afternoon for an hour (local time), Russian media say, though other reports suggest it could be about 30 minutes.
It is unclear if they will speak to reporters afterwards or to what extent media will be admitted to the meeting. After phone calls between the leaders in January and May, the White House and Kremlin released summaries of the conversations.
What we do know is that the two men have staked out opposing views on major international issues in the run-up to the summit:
- On Thursday, Mr Trump used a speech in the Polish capital Warsaw to call on Russia to stop "destabilising" Ukraine and other countries, and "join the community of responsible nations"
- Setting out his own G20 agenda in German financial newspaper Handelsblatt, Mr Putin called for US-led sanctions, imposed in response to Russia's annexation of Crimea in 2014, to be lifted
- Mr Putin also argued strongly in favour of the Paris climate agreement, saying it was a "secure basis for long-term climate regulation" and Russia wanted to make a "comprehensive contribution to its implementation". President Trump, of course, has taken America out of the agreement
While there was less mention of Syria, Washington supports some armed opposition groups, while Moscow is the main ally of President Bashar al-Assad, so the potential for a difference of opinion there too is high.
Mr Trump's choice of words aimed at Russia in his Warsaw speech suggests he recognises the political danger of being seen as too close to Mr Putin and he is already trying to land some blows, the BBC's diplomatic correspondent James Robbins says.
And it remains to be seen whether the issue of Russia's alleged interference in the US presidential election in 2016 will arise.
Will climate change overshadow the summit?
Canadian Prime Minister Justin Trudeau has urged G20 leaders to impress upon Mr Trump that he should act as a role model in addressing climate change.
"We'll tell him it's important to take a lead role in tackling climate change and creating good jobs," he told German tabloid Bild, according to Reuters news agency.
Mrs Merkel has said the G20 will focus on the Paris climate deal but, as the G20 host, she will work to find compromises.
UK Prime Minister Theresa May is expected to challenge President Trump over climate change by saying that the Paris Agreement should not be renegotiated.
Another potential thorny issue at the summit is trade. Mrs Merkel and other EU leaders have expressed their commitment to an open international trading system, while the Trump administration favours protectionism under the "America First" motto.
What about the protests?
Demonstrators could be seen gathering again in Hamburg on Friday morning as businesses surveyed damage to shop fronts and firefighters hosed down freshly burnt cars.
Activists first plan to blockade the city's harbour and the summit venue at Hamburg Messe and Congress Centre.
A march by children and youths is being organised later in the morning and separate gatherings - including a "Sink Capitalism" rally - are also expected.
In Thursday's clashes, which continued into the night, police charged at masked protesters at the "Welcome to hell" march, which was attended by 12,000 people.
Police fired water cannon and pepper spray at masked protesters, who hurled bottles, stones and flares.
Medics were seen treating several people. At least one person appeared to have been seriously hurt and was carried away covered by a foil blanket.