Fico, whose country is the current holder of the EU rotating presidency, told a conference in Bratislava, "I'm not sure whether the UK knows what it wants."
He added, "The split will be painful, but should it be we who suffer? The biggest loss for the EU would be if the UK comes out from the negotiations a winner."
Amid a flurry of Brexit-related activity this week, Belgian Prime Minister Charles Michel has also called on the UK to "clarify" its position on the terms it will seek from the EU "as soon as possible."
RELATED CONTENT
Speaking after a meeting with British Prime Minister Theresa May in Downing Street on Tuesday, Michel said May had confirmed to him that Brexit talks between Britain and the EU are set to start in March.
Following the meeting, he said, "I called for the situation to be clarified as soon as possible, but I have discovered that numerous discussions are still under way here in political circles and in the Westminster Parliament."
But Michel said he remains upbeat, adding, "I have ascertained that the UK government possesses a strong political will to construct a strong economic and political bond with Belgium."
The meeting was the first private conversation between Michel and May since the British voted in June to leave the European Union.
This week, UK Brexit Secretary David Davis was in Strasbourg for a spate of meetings with most of the mainstream group leaders in the European Parliament.
EPP group Chair Manfred Weber, who was among those who met Davis, said, "I have been told that the British government, as far as the economy is concerned, it wants to stay in the single market - but the British government would also like to continue and have very close co-operation in legal issues."
Weber added, "Brexit means Brexit, so we are going to have to cut back on our relationships."
Asked whether the UK aims to remain a full member of the single market, Davis told reporters, "What we are after is that which is in the interest of the [European] Union and in the interest of the UK: trading interests, business, manufacturing and services and the aim is to make it as open as possible…That's the clear overarching aim."