Portaferry athlete Ciara Mageean has failed to qualify for the 1500m semi-final for Team Ireland after coming 10th in the first heat with a time of 4:07.29.
She was four places outside the automatic qualifiers, and lost out on a fastest losers spot.
Mageean was unable to go as far as her semi-final appearance at the Rio games in 2016.
The 29-year-old seemed to be in discomfort in her last lap as she fell down the leaderboard.
"I got in that race, it’s an Olympic Games, you have to make yourself competitive," said Mageean.
"When the race started pressing on and I started pressing that gear, it wasn’t there.
"I’m really disappointed, I’ve proven I’m a world-class 1500m runner and that wasn’t to the level I know I am. I’m not one for making excuses, I didn’t have an ideal week last week. I had a rocky week last week, to say the least.
"I knew the shape was still there, but I wondered if it took the edge off me, the sharpness.
"Things had been very good. I had a tough year, I tore my glute in April time, that put me out for a bit. My last session leading into it, I hurt myself, something happened, and I didn’t get to run Monday to Friday, I ran Saturday and Sunday.
Mageean then revealed she had been struggling with a calf injury even before the games started.
"I tore my calf on Sunday. it was a small thing, normally two weeks and it’d be grand, but I had a week until the Olympic Games.
"I wanted to cry, Sunday, my last 1500m session, no rhyme or reason to it, I have always had achilles issues, not calf.
"It put me out all week, I know I’m in sub-four shape, my sessions have shown that. I don’t get soft tissue injuries, I get chronic tendon injuries.
"But this year has been strange, I tore my glute and the doc was like, 'I’ve never seen this in a runner, never mind an elite runner'.
"It was very strange, on the MRI, all blood and everything. I took it in my stride at the start but as the week went on I got a little anxious, it’s not ideal, you don’t want that going into an Olympic Games.
"I just didn’t have it when the hammer fell. I can’t make a calf heal faster.
"At the beginning of this year where I had a rocky time, Jerry (Kiernan's) death hit me pretty hard, and I still cry, but just willing your body to heal fast, giving it everything you can, knowing the time frame requires a little more.
"Maybe it doesn’t help - I’m a physio and I know that. I put my spikes on for the first time yesterday and it was okay, I can block that (discomfort) out, it wasn’t that in the race, but it was probably the effect of the week."
Ireland teammate, Sarah Healy also competed in the 1500m and finished 11th in her heat in 4:09.78.
NI Commonwealth Bronze Medallist Leon Reid will run in the Men’s 200m in Round 1 on Tuesday, August 3.
Saturday, August 7 will see three of NI’s athletes, Paul Pollock, Kevin Seaward, Stephen Scullion, represent Team Ireland in the Men’s Marathon.
