Mr Tusk made the comments in a letter to the members of the European Council before their meeting at the end of this week.
He said: “The EU and UK negotiators have prepared a joint report on what has been agreed so far, and I think we have a satisfactory result on most issues.
“For that reason, and based on the recommendation by our chief negotiator, I have tabled guidelines which will signal, if you agree, that we are ready to move to the second phase, which will expand discussions to cover transition and the framework for the future relationship.
“The conclusion of the first phase of negotiations is moderate progress, since we only have ten months left to determine the transition period and our future relations with the UK.
“This will be a furious race against time, where again our unity will be key. And the experience so far has shown that unity is a sine qua non of an orderly Brexit.”
In the letter, Mr Tusk said the Council’s unity had been outstanding in the last few months.
Mr Tusk only referred to Brexit in the final paragraph of his open letter to EU leaders.
This comes as the EU Brexit Negotiatior Michel Barnier has said he is ready to present a draft of Brexit withdrawal treaty early next year.
Mr Tusk has said that the UK must respect all of the EU’s laws during the transition period before Brexit comes into force.
More to follow…