The law-and-order candidate of Austria's right-wing party swept the first round of presidential elections on Sunday, winning over 35% of the vote for the party's best ever result.
Government coalition contenders were among the five losers, signalling deep voter rejection and political uncertainty ahead.
The triumph by Norbert Hofer eclipses his Freedom Party's best previous national showing - more than 27% support in 1996 elections that decided Austria's membership in the European Union.
His declared willingness to challenge the governing coalition of centre-left Social Democrats and centrist People's Party spells potential confrontation ahead - Mr Hofer might push for new parliamentary elections should he win the May 22 run-off in hopes that his Freedom Party will triumph.
Preliminary final results with absentee ballots still to be counted gave Mr Hofer 35.5% support, far ahead of Alexander Van der Bellen of the Greens party who ran as an independent. Still, with 20.4% backing, he will challenge Mr Hofer in the second round.
Independent Irmgard Griss came in third. At 18.5%, she was still ahead of People's Party candidate Andreas Khol and Social Democrat Rudolf Hundstorfer, both slightly above 11%. Political outsider Richard Lugner was last, with 2.4%.
With the candidates of establishment parties shut out of the office for the first time since Austria's political landscape was reformed after the Second World War, Freedom Party chief Heinz-Christian Strache hailed the "historic event" that he said reflected massive "voter dissatisfaction".
Still, Mr Van der Bellen remained in the running. Many of those who voted for other candidates are likely to swing behind him in the run-off in hopes he will defeat Mr Hofer and the Freedom Party.
"That was the first round," Mr Van der Bellen said. "The second one will decide."
Mr Hofer's triumph was significant nonetheless, and in line with recent polls showing Freedom Party popularity. Driven by concerns over Europe's migrant crisis, support for his party has surged to 32% compared with just over 20% for each of the governing parties.
But voters were unhappy with the Social Democrats and the People's Party even before the migrant influx last year forced their coalition government to swing from open borders to tough asylum restrictions. Decades of bickering over key issues - most recently tax, pension and education reform - has fed perceptions of political stagnation.
Reflecting voter dissatisfaction, an Orf/Sora/Isa poll of 1,210 eligible voters released on Sunday after balloting ended showed only 19% "satisfied" with the government's work. Its margin of error was 2.8 percentage points.
Vienna Social Democratic Mayor Michael Haeupl spoke of "a catastrophic result," but even worse could lie ahead for both his and the People's Party. As president, Mr Hofer has threatened to call a new national election.
That would likely result in a Freedom Party victory and could move Austria closer to the camp of anti-immigrant Eurosceptic EU nations, further complicating joint European Union attempts to solve the migrant crisis and find consensus on other divisive issues.
An Austrian president has the powers to dismiss a government. But none has since the office was newly defined after the Second World War. Instead, the role has been traditionally ceremonial, with presidents rarely going beyond gentle criticism of the government.
Trying to ease concerns that he would be too confrontational in office, Mr Hofer told reporters that he would be "there for all Austrians".
"No one need be afraid," he told reporters.
Still, he added "that does not mean that I reject my principles". Alluding to his threat, he said that with him as president, the present government would "face serious difficulties" if it didn't change its course.
Political uncertainty may lie ahead, even if Mr Hofer is defeated.
Mr Van der Bellen has vowed not to swear in any Freedom Party politician as Austria's chancellor if he wins the vote.
The president has a six-year mandate. Because parliamentary elections that will decide the next chancellor must be held by 2018, possible confrontation looms between the Freedom Party and Mr Van der Bellen, should he triumph.