EU leaders will start internal talks on trade, paving the way for negotiations to begin, possibly in December.
The PM said there was "some way to go" but she was "optimistic".
The end of the two-day summit in Brussels saw positive comments from some EU leaders and from the prime minister - despite her 27 counterparts ruling out starting formal trade talks with the UK for now.
Mr Tusk told reporters: "My impression is that the reports of the deadlock between the EU and the UK have been exaggerated."
The EU's chief Brexit negotiator, Michel Barnier, described the talks as deadlocked earlier this month.
Mr Tusk said he was not at odds with Mr Barnier, but his own role was to be a "positive motivator for the next five or six weeks".
He said he felt there was "goodwill" on both sides "and this is why I, maybe, in my rhetoric, I'm, maybe, a little bit more optimistic than Michel Barnier, but we are also in a different role".
The so-called divorce bill is a major sticking point in talks with the EU. PM Theresa May declined to say in a press conference after the summit what the UK would be prepared to pay, saying the "final settlement" would come as part of a "final agreement" with the EU.
The prime minister did not name any figures but refused to deny that she had told other EU leaders the UK could pay "many more billions" than the £20bn she had indicated in her Florence speech last month.
"I have said that ... we will honour the commitments that we have made during our membership," she said. But those commitments were being analysed "line by line" she said, adding: "British taxpayer wouldn't expect its government to do anything else."
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