The United States threw down a marker on a dramatic opening night at the World Relays on Saturday, capturing historic gold medals in the mixed shuttle hurdles and 2x2x400 metres.
Devon Allen anchored the Americans to victory over hosts Japan in a shuttle hurdles final reduced to just two teams after Jamaica pulled out due to injuries and Australia's Brianna Beahan was disqualified for a false start.
Rio Olympic finalist Allen powered home on the last leg of the four-lap dash up and down the home straight, winning in 54.96 seconds with Japan clocking 55.59 after an evening of thrills and spills in Yokohama.
Ce'Aira Brown and Donavan Brazier earlier won gold in the mixed 2x2x400 metres, a brutal new event in which two athletes alternately run two legs of 400m each.
Australia's Catriona Bisset and Joshua Ralph took silver with Japan's Ayano Shiomi and Allon Tatsunami Clay collecting bronze.
Team USA had won all six 4x400m at the three previous editions of the World Relays and they set the pace again.
Joanna Atkins and Courtney Okolo brought it home in heat two for the Olympic champions in 3:25.72, comfortably the quickest time going into Sunday's final, with Poland and Canada looking set to contest the silver after Ukraine were DQ'd.
A strong anchor leg from Je'Von Hutchison gave the American men's quartet the fastest time of 3:02.06 as they flexed their muscles ahead of the world championships in Doha this autumn.
Jamaican-born Julian Walsh, the son of a reggae drummer, propelled Japan to victory in heat two with a superb lead-off leg to trigger the biggest cheer of the night.
- Jamaican woe -
"That was pretty much the perfect performance," gushed anchor runner Kota Wakabayashi. "All I had to do was bring the baton home -- these guys did the hard work."
World champions Trinidad and Tobago roared to victory in the third heat to set up a mouth-watering final on the second and last day of the meet.
Britain dominated the men's 4x100m heats as Jamaica continue to struggle without retired sprint king Usain Bolt.
World champion Justin Gatlin helped the Americans clock 38.34 in a fortuitous win aided by a fumbling baton exchange by the Japanese that led to their disqualification. World title holders Britain set the fastest time of 38.11 after a blistering anchor leg from Nethaneel Mitchell-Blake as the Jamaicans trailed in third behind Brazil.
The US women set the pace in the women's 4x100m with Aleia Hobbs bringing the baton home in 42.51, while more butter-fingered chaos by Britain and France gifted Denmark a surprise win and holders Germany powered to victory in their heat.
"It was amazing," said anchorwoman Mathilde Kramer after coming home in a Danish record.
"I was thinking 'Where are all the others, why am I here alone? Okay, just keep running!' I was so excited."
Poland were quickest in the mixed 4x400m heats in 3:15.47, pipping the US, who bucked the trend of starting a male lead-off runner by pitting Brionna Thomas against the men over the first 400.
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