Dina Asher-Smith put down a statement in the women's 100m heats as she ran the quickest time of anyone - 10.84 - to qualify for the semi-finals.
The time was also the second-quickest of the Londoner's career at just one hundredth off her personal best of 10.83.
Asher-Smith stormed out of the blocks and led the race from start to finish in her first outing in Eugene as she looks to add a second world crown to her palmares, following her 200m gold in 2019.
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Speaking to BBC Sport afterwards, she said: "Obviously I'm coming here as a reigning world champion and world silver medallist so clearly the aspirations are high for me.
"I knew that I'd been waiting all season to peak for this. I'm happy, I've got some more to give, but I was happy that I nailed the essential bits.
"I honestly did not expect that time, but at the end of the day, we're looking to go faster so I'm excited."
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Fellow Brit Daryll Neita, who beat Asher-Smith at the UK trials in June, finished her heat second behind Jamaica's Shelly-Ann Fraser-Pryce in a season's best time of 10.95, as she also qualified for the semis.
Neita said: "[I'm] super happy, it's always great to get the first round out of the way and just get a feel of the track and the atmosphere. I'm feeling really excited for the semis.
"I feel like I'm ready for this championships, me and my coach have got me into great shape, so I can really challenge here and I think that opening round shows that."
Meanwhile five British athletes qualified for the 1500m semi-finals, with Laura Muir and Katie Snowden progressing in the women's event and Josh Kerr, Jake Wightman and Neil Gourley doing the same in the men's equivalent.
Muir told the BBC: "It's always a bit nerve-wracking the rounds, you want to think about the final but you've got to get there first.
"It's hard at this level, the strength in depth is really good.
"I was aware there were quite a few of us round there at the end, so I just wanted to stay out of trouble and save as much energy as possible.
"That felt good, that felt comfy."
Kerr won his heat, while Wightman and Gourley finished fourth and fifth respectively.
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