The main leftist opposition party has won the largest share of the vote in the Slovak general election but is far short of an outright majority.
Mr Fico says he wants to push through a left-wing programme
Robert Fico's Smer party has taken 29.14% of ballots against 18.36% for Prime Minister Mikulas Dzurinda's Slovak Democratic and Christian Union.
In order to govern, Mr Fico will have to choose partners from among nearly half a dozen very different parties.
He has promised to roll back many of the centre-right government's reforms.
Saturday's election was the first since Slovakia joined the European Union in 2004.
During his eight years in power, Mr Dzurinda led Slovakia into Nato and presided over strong economic growth but critics say his reforms benefited only the rich.
Admitting defeat, he said that although Mr Fico had "won [the vote], the reforms should continue".
Smer means "direction" in Slovak and Mr Fico wants to change his country's direction, steering Slovakia from right to left, the BBC's Rob Cameron reports from Bratislava.
Mr Dzurinda wanted to continue reforms
The former human rights lawyer, 41, has been riding a wave of public discontent with the right-wing reforms introduced by Mr Dzurinda.
"For us, the result is fantastic," he said.
"I hope that we will succeed in forming a coalition government which will be able to push through a left-wing programme."
Days, weeks, even months of difficult political talks on forming a coalition lie ahead, our correspondent says.
Mr Fico is expected to have 50 seats in the 150-seat parliament to 31 for Mr Dzurinda's party.
Other seats were won by two of Mr Dzurinda's former coalition partners, the Hungarian Coalition Party on 11.69% of the vote, and the Christian Democratic Movement on 8.31%.
The far-right Slovak National Party won 11.73% and the Movement for a Democratic Slovakia took nearly 8.79%, while the Communists failed to clear the 5% hurdle for entering parliament having only won just under 4%.
As Central Europe's longest-serving prime minister, Mr Dzurinda can point to increased international investment and the highest per capita rate of car production in Europe.
However unemployment stands at 15% and many areas away from the capital Bratislava have not experienced strong growth.