Jamaica might have dominated the short sprints and relays in the last few Olympics but they have never won the men's 4x400 metres - something they hope to change on Saturday after edging out the US in their semi-final on Friday.
Jamaica have not even medalled in the race since claiming a silver in 2000 but clocked two minutes 58.29 to beat their big rivals on the line.
The US, stunned by Bahamas four years ago but always the team to beat, have plenty of fresh blood to come in and will start as favourites.
Britain won the second heat impressively ahead of Belgium and defending champions Bahamas but were then disqualified, allowing Brazil to slip into the final by the back door.
Britain's quartet of Nigel Levine, Delano Williams, Matt Hudson-Smith and Martyn Rooney came home first in their semi-final in two minutes 58.88 seconds, establishing themselves as strong medal contenders.
But shortly after leaving the track they were disqualified for starting outside the changeover zone, although it was not immediately clear on which leg the error occurred.
Poland, Cuba and outsiders Botswana also progressed but Trinidad and Tobago were also disqualified after originally finishing in a qualifying position in the first heat.
In the women's 4x400 relay, five-times defending champions the United States and Jamaica, who upset them at last year's world championships, set up what should be a mouth-watering final after impressive semi-final performances on Friday.
Both teams rested key athletes ahead of Saturday's final, aware that their strength-in-depth meant there was no danger of missing out providing there were no baton mishaps.
The Americans, who last missed out on gold in 1992, won the first heat by a distance in three minutes 21.42 seconds and Jamaica nearly matched them in the second with 3:22.38.
Ukraine, Britain, Canada, Italy, Poland and Australia also advanced for what is likely to be a race within a race for bronze.
France and the Bahamas, who set a national record, missed out.
