Tusk, who represents EU leaders on the world stage, said he recommended "that we approve on Sunday the outcome of the Brexit negotiations" in a letter to members of the European Council.
"No-one has reasons to be happy. But at least at this critical time, the EU 27 has passed the test of unity and solidarity," he added.
On Sunday, EU leaders will meet for the special Brexit summit. They will be asked to approve two key Brexit documents: The political declaration, which sets out what the UK and EU's relationship may be like after Brexit - outlining how things like UK-EU trade and security will work.
Besides, the EU withdrawal agreement: a 585-page, legally binding document setting out the terms of the UK's exit from the EU. It covers the UK's 39 billion pound "divorce bill", citizens' rights and the Northern Ireland "backstop" -- a way to keep the border with the Republic of Ireland open, if trade talks stall.
There would not be any formal vote on Sunday but the EU was expected to proceed after reaching a consensus.
Brexit's move forward comes after Spanish Prime Minister Pedro Sanchez received assurances from the UK government over Gibraltar, and dropped his threat to boycott the summit, the BBC reported.
Sanchez received late on Saturday the written guarantees he needed over Spain's role in the future of the British territory.
Even if the EU approves the deal, it still has to be passed by the UK Parliament, with many MPs having stated their opposition.
Former UK Foreign Secretary Boris Johnson, however, has slammed the developements saying the UK would become a "satellite state" under the deal.
UK Prime Minister Theresa May has already arrived in Brussels and has been holding talks with top EU officials, ahead of the summit. She met the president of the European Commission, Jean-Claude Juncker, and Tusk for talks on Saturday evening.
Spain had raised last-minute objections ahead of the summit, but EU leaders secured a compromise with Sanchez saying he would "vote in favour of Brexit", after Europe and the UK "accepted the conditions set down by Spain", the BBC said.
The UK is scheduled to leave the EU on 29 March 2019.
Its withdrawal negotiations have been on since June 2016 following a referendum in which 51.9 per cent voters agreed to leave the EU.