Great Britain's Josh Kerr scraped qualifying for the 1500m semi-finals despite finishing seventh in his heat.
Kerr looked like he would safely progress, but he slowed down at the end and was caught out. His time of 3:36.29 was pipped by Poland's Michal Rozmys (3:26.28).
Nevertheless, he qualified as one of the fastest losers into the semi-finals.
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British number two Jake Wightman is safely through, finishing third from the second heat in a time of 3:41.18 behind Kenya's Abel Kipsang (3:40.68) and American Mathhew Centrowitz (3:41.12).
In the same heat as Wightman, medal contender Marcin Lewandowski from Poland progressed to the semi-finals on appeal after he was tripped up and fell to the ground.
It looked like his Olympics would be over when he got back up to complete his run in last place, but the 2019 World Championships bronze medallist has booked his spot in the next round.
Jake Heyward impressively topped his 1500m heat. His time of 3:36.14 comfortably saw the 22-year-old progress.
In the 400m heats Jodie Williams is easily through as she finished first in her heat in 50.99 seconds, ahead of USA's Quanera Hayes (51.07).
Ama Pipi has qualified as a fastest loser with her time of 51.17 leaving her in fourth after her heat. She was edged out by Czech Republic's Lada Vondrouva in her PB time of 51.14.
American-based Scot Nicole Yeargin looked to have qualified from her heat in third in 52.04 seconds, but she was disqualified for reported lane infringement.
In the men's triple jump, Britain's Benjamin Williams finished 12th in his heat with his 16.30m jump not enough to qualify. Portugal's Pedro Pichardo qualified in first by some margin in 17.71m.
Adam Gemili's Olympic Games are over after he suffered a hamstring injury just before he went out for his 200m heat.
There was also 200m disappointment for Nethaneel Mitchell-Blake. He registered a season best time of 20.56, but he missed out on the top three in heat six in fifth.
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