South Africa’s Wayde van Niekerk completed the first leg of his attempt at a World Championships double with victory in the 400m at London Stadium.
The Olympic champion and world record holder clocked 43.98 seconds to take gold ahead of Steven Gardiner of the Bahamas and Qatar’s Abdalelah Haroun.
Van Niekerk, 25, will continue his 200m campaign in Wednesday’s semi-finals.
Britain’s Kyle Langford narrowly missed out on a surprise medal in the 800m final as he finished fourth.
The 21-year-old from Watford was in last place heading into the home straight but almost overtook Kenya’s Kipyegon Bett on the line for bronze.
Langford set a personal best of one minute 45.25 seconds, with Frenchman Pierre-Ambroise Bosse taking gold and Poland’s Adam Kszczot the silver.
Van Niekerk retained his 400m title in dominant fashion but ran alongside an empty lane as Botswana’s Isaac Makwala was ruled out after an outbreak of the norovirus bug.