The first victory of Sunday's program went to the USA as they won the 4x400m title after a loss to Russia at the 2013 IAAF World Championships.
DeeDee Trotter brought the USA in through the first leg in front (50.7) and handing off to Sanya Richards-Ross should have been enough to build a big lead then. But Jamaica had lined up Novlene Williams-Mills to neutralize Richards-Ross, and she closed the gap on the backstretch and made a race of it, handing off to Anastasia Le-Roy with a slim lead. Jamaica's time at that exchange was 1:40.9, and Williams-Mills' split of 49.7 (unofficially) was the fastest of the race.
Natasha Hastings put the USA back in front immediately on the backstretch of the third leg. Le-Roy held on behind her until the home stretch, where Hastings' pace finally became too much for her. Hastings delivered the stick to Joanna Atkins with 10 metres of space, and Atkins finally put the race away for the USA with a 50.63 split. The USA took the victory in 3:21.73.
Behind, Nigeria, who had been stalking Jamaica all along, closed on Shericka Jackson but weren't able to pull alongside. Jamaica held on to second in 3:23.26, with Nigeria at 3:23.41. Nigeria's team was Folasade Abugan, Regina George, Omolara Omotoso, and Patience Okon George.
Australia won the B final in 3:31.01, but only after duelling the entire last leg with Canada (third, 3:32.58) and holding off a late challenge from Bahamas (second, 3:31.71). The Australians didn't lead the whole race, but were in front when it counted: at every single exchange.
Parker Morse for the IAAF