Thomas Barr ran a season's best in his 400m hurdles semi-final in Tokyo but finished fourth, missing out on an automatic spot in the final.
Barr, starting in lane 6, clocked a time of 48.26, his second fastest ever run, behind only his 47.97 when he finished fourth in the Olympic final in Rio in 2016.
The Waterford sprinter hit a hurdle on the final bend but finished strongly in fourth to leave himself in with a chance of the final, dependent on the times in the two remaining semi-finals.
Italy's Alessandro Sibilio, the third placed finisher in the subsequent semi-final, clocked a time of 47.93 to push Barr out of contention for a final spot.
No joy for Barr. Appeal is rejected due to insufficient evidence. The Italian will not be disqualified. Result stands— David Gillick (@DavidGillick) August 1, 2021
A late appeal from the Irish team over alleged 'hooking' by Sibilo, which would have seen Barr reach an Olympic final for the second time was rejected due to "insufficient evidence".
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The race was won in a time of 47.30 by the current world record holder, Norway's Karsten Warholm.
The winner in the third semi-final, Kyron McMaster of the Virgin Islands, clocked the same time as Barr.
The Irish athlete's time was joint-seventh fastest across the three semi-finals but, for the time being, he now finds himself outside of the final places.
"That's the end of the road," Barr sighed, when speaking to RTÉ Sport immediately afterwards.
"I thought when I hit that hurdle 7 that it was going to cost me. I literally felt like I was in PB shape.
"I was getting into the blocks, thinking yeah, I'm running a PB here and into the final. It cost me a little bit but that's the hurdles. You can win some, you can lose some. There's so many places you can make up time, there's so many places you can lose time.
"Today was just one of those unlucky days.
'Today was just one of those days - I'm probably going to be that 9th or 10th finisher'— RTÉ Sport (@RTEsport) August 1, 2021
Thomas Barr reflects on his #Olympics semi-final elimination #Tokyo2020 #RTESport
?? Report - https://t.co/hQRt2Bqb5d pic.twitter.com/IcVOhZAluA
"The event has moved on so much. There's two guys ready to break the world record, Karsten Warholm has already broke the world record. Two of them were in my race there. It's pushed the whole event on.
"In some ways, it's great. In other ways, it just makes it that bit more difficult to qualify at these championships. That time there would have been good enough to get me into a final of any other Olympics. And possibly even, do well in an Olympic final.
"It's great to be part of it because history is being paved in this event at the moment. It is cool to be part of it. It'd be cool to be part of it to be in a final with them! But just not to be today, it was just a stretch."
The 29-year-old insisted there was more racing in him and admitted it was frustrating that he couldn't deliver his best time on the day.
"That's the frustrating part in that I'm in probably the best shape I've ever been in. And I just didn't get to utilise it fully here today. So, there will be plenty more.
"I always rise for championships. Championships is where I love to come out and challenge and run my best times. It is frustrating and disappointing and I want to thank everyone for all the support. It's been so positive, this whole championship. I'm just glad I didn't get everyone up at a ridiculous hour to watch it!"