What you need to know:
- Turkey's unravelling economy will pose the most immediate test for whoever wins the vote.
Turkey voted Sunday in a historic runoff that President Recep Tayyip Erdogan enters as the firm favourite to extend two decades of his Islamic-rooted rule to 2028.
The NATO member's longest-serving leader defied critics and doubters by emerging with a comfortable lead against his secular challenger Kemal Kilicdaroglu in the first round on May 14.
The vote was nonetheless the toughest the 69-year-old has faced in one of Turkey's most transformative eras since its creation as a post-Ottoman republic 100 years ago.
Kilicdaroglu cobbled together a powerful coalition of Erdogan's disenchanted former allies with secular nationalists and religious conservatives.
Opposition supporters viewed it as a do-or-die chance to save Turkey from being turned into an autocracy by a leader whose consolidation of power rivals that of Ottoman sultans.
But Erdogan still managed to come within a fraction of a percentage point of winning outright in the first round.
His success came in the face of one of the world's worst cost-of-living crises, with almost every opinion poll predicting his defeat.
Waiting outside an Istanbul polling station on Sunday, 93-year-old Ozer Atayolu told AFP he always arrived first to vote "because I believe in democracy and my responsibility as a citizen".
"I feel like a child having fun," the retired textile engineer said.
Kilicdaroglu tried his best to keep his disappointed supporters' spirits up.
"Do not despair," he said on Twitter after the vote. But he then vanished from view for four days before re-emerging a transformed man.
The former civil servant's old message of social unity and democracy gave way to desk-thumping speeches about the need to immediately expel migrants and fight terrorism.
His right-wing turn was targeted at nationalists who emerged as the big winners of the parallel parliamentary elections.
The 74-year-old had always adhered to the firm nationalist principles of Mustafa Kemal Ataturk, the military commander who formed both Turkey and Kilicdaroglu's secular CHP party.
But these had played a secondary role to his promotion of socially liberal values practised by younger voters and big-city residents.
Analysts question whether Kilicdaroglu's gamble will work.
His informal alliance with a pro-Kurdish party left him exposed to charges from Erdogan of working with "terrorists".
The government portrays the Kurdish party as the political wing of outlawed militants.
And Kilicdaroglu's courtship of Turkey's hard right was hampered by the endorsement Erdogan received from an ultra-nationalist who finished third two weeks ago.
'Transactional and tense'
The political battles are being watched closely across world capitals because of Turkey's footprint in both Europe and the Middle East.
Erdogan's warm ties with the West during his first decade in power were followed by a second in which he turned Turkey into NATO's problem child.
He launched a series of military incursions into Syria that infuriated European powers and put Turkish soldiers on the opposite side of Kurdish forces supported by the United States.
His personal relationship with Russian President Vladimir Putin has also survived the Kremlin's war on Ukraine despite Western sanctions against Moscow.
Turkey's troubled economy is benefiting from a crucial deferment of payment on Russian energy imports, which helped Erdogan spend lavishly on campaign pledges this year.
Erdogan also delayed Finland's membership of NATO and is still refusing to let Sweden join the US-led defence bloc.
The Eurasia Group consultancy said Erdogan was likely to continue trying to play world powers off each other should he win.
"Turkey's relations with the US and the EU will remain transactional and tense," it said.
'Day of reckoning'
Turkey's unravelling economy will pose the most immediate test for whoever wins the vote.
Erdogan went through a series of central bankers until he found one who started enacting his wish to slash interest rates at all costs in 2021 -- flouting the rules of conventional economics in the belief that lower rates can cure chronically high inflation.
Turkey's currency soon entered a freefall and the annual inflation rate touched 85 percent last year.
Erdogan has promised to continue these policies, despite predictions of economic peril from analysts.
Turkey burned through tens of billions of dollars while trying to support the lira from politically sensitive falls ahead of the vote.
Many analysts say that Turkey must now either hike interest rates or abandon its attempts to support the lira -- two solutions that would incur economic pain.
"The day of reckoning for Turkey's economy and financial markets may now just be around the corner," analysts at Capital Economics warned.