Andrew Coscoran's first Olympics is over as the pace proved too hot down the straight in his 1500m semi-final with Britain's Jake Wightman winning in 3:33.48, ahead of USA's Cole Hocker and Kenya's world champion Timothy Cheruiyot.
Balbriggan man Coscoran ran a time of 3:35.84, which was just outside his personal best of 3:35.66 and in 10th place in the semis.
There was a big surprise in the second-semi-final with the Rio 1500m gold medal winner Matthew Centrowitz of USA failing to make it into the final to defend his crown.
Centrowitz finished in 9th place in a race won by Kenya's Abel Kipsang.
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Speaking to RTÉ Sport, a pleased and tired Coscoran said: "I ran 3:35.8, which is pretty good, but I'm still way off! The standard is just insane. There are some class guys in there. I'm happy to be in the mix with them.
"I knew it was quick, so I went to the back and waited for guys to die off. Everyone was able to hold it together and I wasn't able to come through like I wanted to.
"I always knew I could ran faster than my personal best, but to get so close to it and after a race two days ago and the heat here and the pressure, I'm pretty happy. I'm surprised that 3:35.8 gets you nowhere near the final, it's madness, the standard is crazy."
The Paris Olympics in three years' time is now a target for the promising runner who hopes to get more opportunities at world level in order to improve.
He concluded: "This is my first world senior event. I've done European indoors. But this is the first world stage and I've made the semis. I'm chuffed, and going forward I think I can get into some of the bigger races like the Diamond League.
"This season I was struggling into some of the bigger races so hopefully this time they let me in."