But Tusk said there was some positive news out of Thursday's meeting between Johnson and Irish Prime Minister Leo Varadkar.
This takes the negotiations into "the tunnel", which is Brussels jargon for an in-depth phase of talks among a select group of negotiators created to minimise leaks.
Documents are not published, press reaction or comment is off-limits and even the ambassadors of the EU-27 are not kept informed of progress in the regular way that is usual.
The EU's position remains the same: there must be a legally operative solution in the Withdrawal Agreement that avoids a hard border on the island of Ireland, protects the all-island economy and the Good Friday (Belfast) Agreement in all its dimensions, and safeguards the integrity of the Single Market.
He had earlier cautioned that Brexit was "like climbing a mountain", saying that both sides needed "vigilance, determination, and patience".
Tusk said that he had been prepared to announce publicly on Friday that there were "no more chances" for a Brexit deal that could be agreed at the upcoming meeting of European leaders.
A statement from the UK Government also described it as a "constructive meeting".
Dramatic progress could lead to the start of so-called intensive "tunnel" negotiations in the coming days, ahead of the crunch European Union heads of government summit.
The pound rose for the second day on global money markets as traders welcomed signs that a Brexit deal may be possible.
Meanwhile the DUP, whose support is likely to be crucial if Mr Johnson is to get a deal through Parliament, warned they would only back measures that were in the "long-term economic and constitutional interests " of Northern Ireland.
Anonymous Downing Street sources accused Mr Varadkar of backtracking on previous commitments to try to find a deal, and of refusing to negotiate.
After days of signals that the talks were close to collapse, there is now speculation of a United Kingdom compromise on the issue of the backstop, created to prevent a hard border on the island of Ireland.
He said the United Kingdom had yet to put forward an "operational, legally binding solution" to replace the Northern Ireland backstop - meant to prevent the return of a hard border with the Republic.
The PM has said that, while he will abide by the law, he is determined to leave on the Halloween deadline of October 31 come what may.
Many MPs believe if he can not get a deal, Mr Johnson will use the occasion to push for a "people versus Parliament" general election, possibly as early as next month.
'And, you know, if they can't then we have to be ready, as this country is and will be, to come out with no-deal if we absolutely have to'.