The face-off, on January 26-27, is shaping up to be a clash not only of politics, but also personalities.
Zeman's most serious challenger is Jiri Drahos, 68, a chemical engineer and former head of the Czech Academy of Sciences who has campaigned on anchoring the Czech Republic's place in Europe.
Zeman has courted controversy by voicing antimigrant views, denigrating Muslims, and warming up to Russian President Vladimir Putin at a time when many in Europe fear that Moscow is meddling in Western elections and affairs.
Initial results indicating which two candidates are likely to contest the expected run-off are expected later Saturday.
With almost all the votes counted, Zeman was credited with 38.57 percent of the vote in the election, held Friday and Saturday, with Drahoš at 26.6 percent, well ahead of third-placed Pavel Fischer, a former ambassador to France, who garnered just over 10 percent.
"If Drahos wins, the image of the president will be different than in the Zeman era because Drahos is an intellectual without a radical vision or appeals", said Lubomir Kopecek, a political analyst at Masaryk University in the city of Brno.
Meanwhile, Mr Zeman's hardline approach to immigration from Muslim-majority countries has won him favour from large segments of the Czech public.
His more liberal rival Jiří Drahoš is staunchly pro-European and has called for Prague to "play a more active role in the EU".
Zeman's rhetoric and actions has provoked the ire of many Czechs, particularly in the capital, Prague, where Zeman cast his ballot Friday.
"The president should work to unite society".
Songwriter and businessman Michal Horacek, 65, could also vie for a spot in the run-off while support has risen for former center-right Prime Minister Mirek Topolanek and diplomat Pavel Fischer.
Zeman says he is ready "to meet (Drahos') request" to face each other.
"There is, I would say, a lot of public interest in this as it is very much related to the current president who managed to divide our society", said Dita Charanzova, a member of European parliament who is also an adviser to caretaker Prime Minister Andrej Babis, a Eurosceptic populist and billionaire.
Babis's populist ANO movement won last October's general elections with its anti-corruption and anti-euro campaign, but the Slovak-born tycoon facing police charges over European Union funding fraud failed to woo coalition partners.
Zeman, 73, was elected in 2013 during the country's first direct presidential vote, a victory that returned the former left-leaning prime minister to power.
While Zeman's backing for Babis may prolong political uncertainty as the cabinet is expected to lose a confidence vote next week, Czech financial assets have been largely immune.
But the situation could change dramatically if Drahos wins.
The CTK news agency pegged turnout at 40% after day one of voting. "I'm still young, full of strength and full of energy and I look forward to debate", he remarked, while urging supporters to bring along their close ones, including "relatives and lovers", to the second-round polling stations.