The U.S. came close to botching changeovers in both the men's and women's heats but still qualified for both finals. Michael Rodgers slowed to a near-halt before his changeover with last-leg runner Cravon Gillespie, who had to sprint hard to make the final behind Britain and Brazil. South Africa won the other men's heat in a new African record time.
In the women's heats, Dezerea Bryant and Teahna Daniels of the U.S. struggled to hand over the baton, but eventually made it work as their team won the heat in 42.46 seconds ahead of Trinidad and Tobago.
Australian women's relay hopes ended when third-leg runner Maddie Coates fell and took teammate Nana Adoma Owusu-Afriyie down with her. France and Brazil were disqualified. Jamaica qualified fastest from the women's heats in 42.11 in part thanks to a fast leg from 100-meter world champion Shelley-Ann Fraser-Pryce. Britain was second and China third.
The finals are Saturday.
Timothy Cheruiyot of Kenya secured his place in the men's 1,500-meter final in front of the largest crowds yet at the Doha track world championships.
The silver medalist in 2017, Cheruiyot won his semifinal in 3 minutes 36.53 seconds in a race where Norway's European champion Filip Ingebrigtsen finished just seventh and failed to qualify.
Poland's Marcin Lewandowski won the other heat in 3:36.50 ahead of Kenyan Ronald Kwemoi and Ingebrigtsen's brother Jakob.
Defending champion Elijah Manangoi of Kenya is not racing due to an ankle injury.
The Khalifa International Stadium has its biggest crowds of the championships so far, drawn by ticket giveaways and the promise of watching Qatar's Mutaz Essa Barshim contend for a medal in the men's high jump.
After days when showpiece events like the men's and women's 100 finals were greeted by rows of empty seats, places are so in demand that organizers have removed awnings over sectors of the stadium to increase capacity.
The world record could fall as Dalilah Muhammad and Sydney McLaughlin of the United States do battle once again in the women's 400-meter hurdles on day eight of the world track and field championships.
Muhammad set a new world record of 42.20 seconds when she beat McLaughlin at the U.S. nationals in July. McLaughlin was the faster of the two Wednesday as both won their semifinals.
Also Friday, Fred Kerley of the U.S. is the favorite for the men's 400 after fellow American Michael Norman was eliminated in the semifinals. The 2011 world champion Kirani James of Grenada could also challenge for gold.
Medals are also on offer in the men's high jump and women's discus, as well as the men's 3,000-meter steeplechase and 20-kilometer race walk. The men's and women's 4x100 relays start with heats.
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