American Noah Lyles shot to 200 metres world championship gold on Tuesday, providing a bright finish to a day darkened by yet another doping scandal.

The showman Lyles, his hair dyed silver after a heroic animee character, won the first of what many predict will be many world championship medals by storming across the line unthreatened in a time off 19.83 seconds.

Canada's Andre De Grasse, the Rio Olympic silver medallist behind Usain Bolt, was second again in 19.95 while Ecuador's Alex Quinonez took the bronze by clocking 19.98.

It was a brilliant finish to a day of competition clouded by a drugs controversy that began with renowned American coach Alberto Salazar being banned for four years for doping violations.

Salazar, the head coach at the Nike Oregon Project, was banned for four years by the U.S. Anti-Doping Agency for "orchestrating and facilitating prohibited doping conduct".

Salazar has denied wrongdoing and vowed to appeal and sportswear giant Nike has said it will stand by the 61-year-old coach.

Donavan Brazier, who trains at the Nike base, stormed home and broke the world championship record to claim a first-ever world title for the United States in the men's 800 metres.

The 22-year-old, running in his first world or Olympic final, destroyed the field as he raced past Puerto Rican Wesley Vazquez with 300 metres left to win in one minute 42.34 seconds.

Bosnia's Amel Tuka, who won the bronze in Beijing in 2015, went one better by taking the silver and Kenya's Ferguson Cheruiyot Rotich got the bronze.

The race was left wide open after Botswana's Nijel Amos, one of the favourites, withdrew before his opening heat on Saturday with an Achilles injury and titleholder Pierre-Ambroise Bosse was eliminated in the semi-finals.

Vazquez quickly took the initiative and led at the bell with Brazier tucked in behind and Tuka in third.
There was never any doubt above the outcome when Brazier made his move down the back straight while Vazquez was also overhauled by Tuka and Rotich on the final bend.

Sam Kendricks of the United States clinched his second consecutive world title on Tuesday after a duel with European champion Armand Duplantis that electrified the usually muted crowd at Khalifa International Stadium.

Kendricks stayed alive by clearing 5.87 and 5.97 metres in his third and final attempts but it was his first-time clearance at 5.92 that put him above Duplantis and earned him gold when neither man could clear 6.02.

Kendricks, who came into the championships with a season's best of 6.06, became the first pole vaulter to repeat at the worlds since Sergey Bubka clinched his sixth consecutive title in 1997.

Dina Asher-Smith (C) of Britain in action during the women's 200m semi finals

Britain's Dina Asher-Smith led the charge into the women's 200 metres final while Jamaican Olympic champion Elaine Thompson pulled out to leave  the gold medal up for grabs.

A silver medallist in the 100m, Asher-Smith will go into Wednesday's final as the clear favourite for gold after cruising home in 22.16 seconds ahead of Americans Brittany Brown (22.46) and Anglerne Annelus (22.49) who posted the top times in the other two heats.

The final will be without both the defending world and Olympic champions in an event that was depleted by withdrawals.

Thompson, who narrowly missed out on a 100m medal on Sunday by finishing fourth, was the latest casualty when she withdrew from the semi-finals.

The 200m has been a double disappointment for Jamaica with Thompson following the withdrawal of 100m champion Shelly-Ann Fraser-Pryce, who scratched from the opening heats with soreness.

Also failing to reach the starting blocks for the first heats were Ivory Coast's 100m bronze medallist Marie-Josée Ta Lou and Dafne Schippers, 200m champion at the last two worlds, who pulled out with strained abductor muscle.

The event had already lost one of the gold medal favourties Shaunae Miller-Uibo of the Bahamas. She owns the best time of 21.74 this season but opted out of the 200m in Doha to run the 400m.