Ciara Mageean ran her fastest ever 1500m season opener at Friday's Diamond League meeting in Florence.
The two-time Olympian finished fourth in a time of 4.00.95, four seconds outside her national record time of 3:56.63.
It was a race which saw Kenya's Faith Kipyegon bury the ghost of past disappointments with an emphatic performance, nearly nine months after missing out on the 1500m world record by three-hundredths of a second.
In Monaco last year, double Olympic and world 1500m champion Kipyegon came agonisingly close to breaking Genzebe Dibaba's mark of 3:50.07, set in 2015, as she crossed the line in 3:50.37.
However, on Friday, Kipyegon did not let up for a moment, doggedly continuing on long after she had left her rivals in the dust to cross the line in 3:49.11 - almost an entire second faster than Dibaba.
"When I crossed the finish line, I knew I broke the world record because I had a good finish and felt very strong at the end. I knew everything was possible," Kipyegon said after her remarkable run on Friday.
The 29-year-old Kenyan remained behind the pacemakers for the first half of the race before running clear in the final 300m to finish well ahead of Britain's Laura Muir and Australian Jessica Hull.
"After 1,000, when the pacemaker went out, I just pushed myself towards the finish line. And that was what my manager told me - anything is possible - after the pacemaker, just run your race," Kipyegon said.
The world record completes Kipyegon's collection of achievements, adding to her two Olympic 1500m golds and the 2017 and 2022 world titles, Commonwealth Games gold medal from 2014 and three Diamond League titles.
"This was really important because this was something I was still missing in my career," Kipyegon added.
"Getting this, it will really motivate me and I left the legacy for the next generation - they can say she broke the world record, she was the Olympic and the world champion. It was amazing tonight."
Additional reporting: Reuters
