Noah Lyles conceded he was shocked after being beaten for the first time in nearly three years by fellow American Michael Norman at the men's 200m in Rome's Diamond League meeting on Thursday.
Norman, 21, got off to a blistering start and held on despite a late surge by Lyles, also 21, in Rome's Olympic Stadium to achieve a world leading time for the season of 19.70sec.
Lyles, winner of the past two Diamond League trophies over the distance, posted 19.72sec, with Ecuador's Alex Quinonez a distant third in 20.17sec.
World champion Ramil Guliyev of Turkey trailed in fourth in 20.35sec ahead of his title defence at the world championships in Doha in September.
"This race didn't really go as I expected," said Lyles, who had not been beaten since the US trials in July 2016 which saw him fail to qualify for the Rio Olympics.
Since then he has claimed nine straight victories and dominated the Diamond League in 2017 and 2018, but missed the world championships two years ago due to an injury.
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"Winning is what is always expected, but I ran faster than in the last race so I can feel great things will happen," he said.
"The next competitions are in the USA, and before thinking of the World Championships, I have to make the team. It's really a question of making or not making the team.
"I need to stay healthy, improve my start, as much my finish, so I can come out strong."
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