Erdogan, 69, defied opinion polls and came out comfortably ahead with an almost five-point lead over his rival Kemal Kilicdaroglu in the first round on May 14. But he fell just short of the 50 per cent needed to avoid a runoff, in a race with profound consequences for Turkey itself and global geopolitics.
His unexpectedly strong showing amid a deep cost of living crisis, and a win in parliamentary elections for a coalition of his conservative Islamist-rooted AK Party (AKP), the nationalist MHP and others, buoyed the veteran campaigner who says a vote for him is a vote for stability.
Kilicdaroglu, 74, is the candidate of a six-party opposition alliance, and leads the Republican People's Party (CHP) created by Turkey's founder Mustafa Kemal Ataturk. His camp has struggled to regain momentum after the shock of trailing Erdogan in the first round.
The election will decide not only who leads Turkey, a NATO-member country of 85 million, but also how it is governed, where its economy is headed after its currency plunged to one tenth of its value against the US dollar in a decade, and the shape of its foreign policy, which has seen Turkey irk the West by cultivating ties with Russia and Gulf states.
The initial election showed larger-than-expected support for nationalism - a powerful force in Turkish politics which has been hardened by years of hostilities with Kurdish militants, an attempted coup in 2016 and the influx of millions of refugees from Syria since war began there in 2011.
Turkey is the world's largest host of refugees, with some five million migrants, of whom 3.3 million are Syrians, according to Interior Ministry data.
Third-place presidential candidate and hardline nationalist Sinan Ogan said he endorsed Erdogan based on a principle of "non-stop struggle (against) terrorism", referring to pro-Kurdish groups. He achieved 5.17 per cent of the vote.
Another nationalist, Umit Ozdag, leader of the anti-immigrant Victory Party (ZP), announced a deal declaring ZP's support for Kilicdaroglu, after he said he would repatriate immigrants. The ZP won 2.2 per cent of votes in this month's parliamentary election.
A closely-watched survey by pollster Konda for the runoff put support for Erdogan on 52.7 per cent and Kilicdaroglu on 47.3 per cent after distributing undecided voters. The survey was carried out on May 20-21, before Ogan and Ozdag revealed their endorsements.
Another key is how Turkey's Kurds, at about a fifth of the population, will vote.
The pro-Kurdish Peoples' Democratic Party (HDP) party endorsed Kilicdaroglu in the first round but, after his lurch to the right to win nationalist votes, it did not explicitly name him and urged voters rather to reject Erdogan's "one-man regime" in the runoff.
Polls close at 0000 AEST. By late on Sunday there should be a clear indication of the winner.
