Croatia's conservative president trailed her leftist rival in Sunday's election, but garnered enough votes to force a runoff early next year.
With nearly 98% of the votes counted, former prime minister and leader of the Social Democrats Zoran Milanovic was leading with nearly 30% of the vote, followed by President Kolinda Grabar Kitarovic with about 27%.
Milanovic and Kitarovic will face each other on January 5.
Croatia's presidency is largely ceremonial but retaining the presidency is important to Kitarovic's ruling Croatian Democratic Union party because the country is set to take over the European Union's rotating presidency in February.
In that capacity, the Croatian leader will oversee Britain's exit from the bloc and the post-Brexit trade talks that will determine the union's future.