The two leaders have agreed to "lower tensions" over the UK overseas territory of Gibraltar in the talks, a source told the Independent newspaper.
Gibraltar, the United Kingdom's overseas territory in the south of Spain and claimed by the latter, became the source of tensions between Madrid and London after Brussels suggested last week that Spain could be granted a veto right over Brexit terms concerning Gibraltar.
The United Kingdom officially began the EU withdrawal process in late March by handing a formal letter to Tusk. According to article 50 of the Lisbon Treaty, the United Kingdom now has two years to complete the negotiation process.
Gibraltar, a home to some 32,000 inhabitants, was ceded to Britain in 1713 but Spain continues to lay claims to the territory on its southern tip.