Rival cities Tokyo, Madrid and Istanbul are making their final pitches to host the 2020 Olympic Games before members of the International Olympic Committee cast their deciding votes in Buenos Aires, Argentina, on Saturday.
The race is expected to go to the wire, with all three contenders dogged by serious doubts and setbacks they have struggled to overcome.
The prime ministers of all three countries, Shinzo Abe of Japan, Mariano Rajoy of Spain and Recep Tayyip Erdogan of Turkey, flew to Buenos Aires from the G-20 summit in St Petersburg to lend backing to their bids.
Bookmakers said they experienced a surge of late interest in Madrid, with its odds slashed to 5/4, from 4/1 a week ago.
Tokyo remains the favourite, though its odds have shortened to 5/6. Istanbul is listed at 6/1.
Tokyo had pitched itself as the "safe" choice, but its bid was thrown into turmoil by renewed concerns over radioactive leaks at the crippled Fukushima nuclear plant.
Madrid, previously written off due to the country's financial troubles, has generated the most buzz and momentum in recent days, and could be poised for an upset win.
Istanbul's prospects have been hampered by the escalating conflict in neighbouring Syria, as well as anti-government protests in Turkey, and a string of sports doping scandals.
However, it too enjoyed a late surge of its own, with the hashtag #istanbul2020 trending across the world on Twitter.
"It's certainly an open race," said South African IOC member Sam Ramsamy. "They all have positive and negative points. The final presentations will be crucial."
IOC elections are notoriously unpredictable, with the secret ballot characterised by much horse-trading and arm-twisting.
Some members remain undecided and will listen closely to the final presentations before casting their votes.
All 97 IOC members will be eligible to vote in the first round, with the process likely to go into a second round before either city can win a majority.
The city with the fewest votes is eliminated after the first round, setting up a final head-to-head ballot.
IOC president Jacques Rogge will then open a sealed envelope to announce the winner.
All three cities are repeat bidders. It is Istanbul's fifth attempt, Madrid's third consecutive bid, and Tokyo's second successive effort.
Istanbul is pitching its case as a "historic choice" for the IOC, with a win for the city, which straddles Euope and Asia, heralding the first Olympic Games in the Middle East and the first in a predominantly Muslim nation.