The next round of talks on a transition period after the UK leaves in March 2019 could begin as early as next week.
European Commission President Jean-Claude Juncker said this process would be "significantly harder".
Mrs May was applauded by other leaders at dinner in Brussels on Thursday night after she made a speech urging the two sides to embrace the way ahead with "creativity and ambition".
The European Commission has said "sufficient progress" has been made on the first phase to move onto discussing the framework of a future relationship - including issues such as security and trade.
Mr Juncker said the EU's initial priority was to "formalise the agreement we have now" on issues such as citizens' rights before starting negotiations on a future relationship after the UK leaves in March 2019.
"The second phase will be significantly harder than the first and the first was very difficult," he warned.