With nearly 90 percent of the ballots counted by Monday afternoon, Volodymyr Zelenskiy, 41, had a 14 point lead on President Petro Poroshenko, beating all expectations.
Talking to cheering supporters after his lead became apparent on Sunday, Zelenskiy thanked Ukrainians for not casting their ballots "just for fun."
"This is only the first step toward a great victory," said Zelenskiy, who has never held political office.
The comedian, who led opinion polls throughout the campaign, has vowed to enact anti-corruption reforms and make Ukrainians "prosperous." He has also promised to end the war with pro-Russian separatists in the east of the country.
“You don’t need experience to be president, you just need to be a decent human being," was a mantra repeated by him and his aides.
Zelenskiy and Poroshenko looked set to clash in the run-off on April 21, when things could get heated, according to a Kiev-based political strategist, Olexiy Golobutskiy.
Let our news meet your inbox. The news and stories that matters, delivered weekday mornings.
The race will likely become "much more rough" in this next stage of campaigning, Golobutskiy said. "It’s not easy to make up a gap of more than 10 percent."
Most opinion polls released before the election predicted that Zelenskiy would beat any candidate who faces him in the runoff.
Golobutskiy said Poroshenko and his team will have to dig deep to try to lure voters who gave their votes to the candidates who did not make it to the second round. There were originally 39 candidates on the ballot.
“Poroshenko’s campaign will also be faced with the task of demotivating the voters who voted for Zelenskiy," Golobutskiy said.
Emily Ferris, a Russia research fellow at the London-based think tank Royal United Services Institute, said voters’ behavior can be unpredictable and many may opt out for continuity when faced with a choice between Poroshenko and "an unknown alternative" like Zelenskiy in the runoff.
Poroshenko, 53, has tried to convince voters with a militarist message. Casting himself as the man who can prevent Ukraine from becoming a Russian vassal state, he has also promoted the Ukrainian language and been instrumental in establishing a new independent Orthodox church.
But his inability to tackle rampant corruption and plummeting living standards had prompted many to consider alternatives.
Poroshenko called the result a "stern lesson," saying that he has heard loud and clear the "signal" sent by the voters on Sunday.
He then called Zelenskiy "a puppet of oligarch Igor Kolomoyskyi," and said that Russian President Vladimir Putin "dreams of a soft, meek, compliant, giggling, inexperienced and weak, ideologically amorphous and politically undecided president of Ukraine."
Balázs Jarábik, a nonresident scholar at the Carnegie Endowment for International Peace, predicted that Poroshenko will continue portraying Zelenskiy as someone who will cozy up to the Kremlin in the weeks to come.
Zelesnky had previously said that he was ready to negotiate directly with Putin if that meant ending the conflict in the east.
"Such a polarizing campaign, though, is one of the reasons why many Ukrainians support Zelenskiy," Jarábik added.
Despite some reports of irregularities, most experts and observers deemed Sunday's election transparent and fair, with over 63 percent turning up to vote — higher than after the 2014 protests that shook the country.
"Many Ukrainians felt that saying 'No' to the old system, represented by Poroshenko and Tymoshenko, is very important,” he said, referring to Yulia Tymoshenko, the nationalist former prime minister who placed third with 13.2 percent.
The result "is very good news for Ukraine's democracy," he added.