Just when it looked like a clear two-city race, bookmakers have seen late money for Madrid to spring a surprise by winning the vote to stage the 2016 Olympcs.
Chicago remains a firm favourite following Barack Obama's decision to travel to Copenhagen, closely followed in the betting by Rio de Janeiro.
However, Ladbrokes report some last-minute support for the Spanish capital to sneak up on the rails unnoticed while the big two battle it out.
"Chicago remains the favourite but punters, buoyed by London's shock victory last time, have switched their attention to one of the outsiders," said spokesman Nick Weinberg.
Their odds on Madrid getting the nod have been cut from 16/1 to 9/1, while the Windy City heads the market on 4/6, Rio has been eased out slightly to 7/4 and Tokyo is now a longshot at 25/1.
Coral also still have the American bid as clear favourites at 8/11 ahead of tomorrow's announcement in Denmark.
"Chicago has always been favourite to win the vote," said spokesman David Stevens. "And the news that President Obama will be flying in to support the bid saw their odds shorten further."
William Hill, meanwhile, are offering 9/4 that Chicago hosts these Games and Obama is still president at the time they take place.
"Chicago has attracted all the serious money, particulary since it was announced that Obama was flying to support their bid," said spokesman Graham Sharpe. "Punters obviously believe it's a done deal.
"We have seen a little support for Madrid in the last day or so and their odds have been cut from 16/1 to 10/1, but if money talks the Windy City will breeze in."
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