Nearly exactly a year after Britain's seismic referendum to leave the bloc, the EU's chief negotiator, Michel Barnier of France, welcomed his counterpart David Davis with a cheery handshake at the European Commission in Brussels.
It will test the ingenuity of thousands of public servants racing against the clock to untangle 44 years of European Union membership before Britain is out, 649 days from now, on March 30, 2019. Barnier said they wanted to agree on the "main principles of the key challenges of the UK's withdrawal as soon as possible", including Britain's exit bill, the rights of European Union citizens in Britain and the future of Northern Ireland.
Mr Davis's meeting with the administration of prime minister Mariano Rajoy came after a setback in Brussels saw United Kingdom officials agree to the Brexit timetable set out by the European Commission's chief negotiator Michel Barnier.
A key issue he did not mention was the EU's bill for Britain to leave, which Brussels estimates at a colossal 100 billion euros.
As with Monday's seven hours of meetings between Barnier, Davis and their teams, her emphasis is expected to be on easing tensions that have seen some spiky exchanges in recent months.
Both EU negotiator Michel Barnier and his British counterpart David Davis said after the first negotiating session they were confident of quick progress but said major challenges lay ahead to meet the deadline of March 2019 for Britain to officially leave the bloc.
"While there is a long road ahead, our destination is clear - a deep and special partnership between the United Kingdom and the European Union", he said in a statement.
European stocks rose on Monday, partly on optimism about the talks actually getting underway after months of sniping and uncertainty, analysts said. And almost three months have ticked away since Prime Minister Theresa May started the two-year countdown to departure.
"I would like us to get a good agreement that is in both sides' interests".
Anxious by immigration and loss of sovereignty, Britain voted a year ago to end its four-decades-old membership of the 28-country bloc - the first state ever to do so. "We must think about the deep and meaningful partnership with Europe", he said.
But the situation is very different from 12 months ago when the Brexiteers were riding high, with Prime Minister Theresa May's entire approach called into question after a disastrous election performance on June 8.
With barely 21 months left until the deadline to reach a deal on the first ever departure of an European Union country, negotiators said the first full round of talks would not start until July 17 - a sign that pressure is likely to mount as they confront a dizzying array of complex issues, including citizens' rights and a financial settlement.
As the Europe's chief negotiator, French Republican Michel Barnier will lead the EU's side of discussions as dictated to him by the 27 heads of member states, as well as European Council president Donald Tusk.
"The best way we can spend this week is to rebuild trust", rather than tackle the big hard issues right at the start, another European source said.
"In the first step, we will deal with the most pressing issues".
"So it's in this acrimonious environment that the process begins and that is going to make it tough going for investors looking for clues about whether we get a soft or hard Brexit, something that is between the two, or no Brexit".