Yet it was the opposite story for her Irish teammate Sophie O’Sullivan, with the NCAA champion pulling out a theatrical dive across the finish line to grab the last qualifying spot from her heat.

The duo’s performances lifted the Irish spirits on the opening day of action at the Japan National Stadium, following the failure of the Irish mixed 4x400m team to reach the final earlier in the day.

Healy and O’Sullivan had come to Tokyo with contrasting build-ups. While Healy has been a model of consistency all season, O’Sullivan had a troubled preparation.

Not long after her victory at the NCAA Championships in June, the 23-year-old missed a block of training due to illness, and things went from bad to worse in July when she was diagnosed with a stress fracture in her tibia, which kept her sidelined for about three weeks.

She clocked a pedestrian 2:11 for 800m in her final tune-up race which left her full of doubt. “I was like, ‘Jeez, what’s the point in even coming out here?’” she said. “But I just had to forget about that.”

She needed a top-six finish to advance to Sunday’s 1500m semi-finals, and the University of Washington graduate played a patient game in her heat, tracking the leading six. She got trapped in a box on the final bend and needed to utilise her vast range of gears in the last 100m, moving from seventh to sixth in the dying strides and crashing to the track to secure the final automatic spot in 4:02.12.

“I got a little bit caught up, as I seem to always do, and I thought, ‘F*** it, I’m not getting stuck out of this again, I’m going to go around everyone – do whatever it takes,” said

O’Sullivan, who narrowly missed the semi-finals at the Paris Olympics. “I honestly went all in, lost my legs, and I thought if I’m going to fall I may as well get a bit of forward momentum with the fall. I’m lucky.”

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For Healy, the route to the semis was much smoother, the 24-year-old running close to the front and avoiding trouble through the first three laps.

She kept her cool on the final bend as a cluster of athletes surged up behind, Healy changing gears impressively to ease across the line in third in 4:02.67, with Olympic champion Faith Kipyegon winning in 4:02.55.

“It was good, it was as smooth as you can hope for,” she said. “I’m super relieved to qualify safely. It’s always nerve-wracking but I think it went pretty well. There were so many there with 150m to go and I was following Laura [Muir] and I sensed she was coming back to me. The last 30 metres, I felt like I was safe but you never know, so I just kept going.”

Healy came into the event ranked 10th and she will need a top-six finish in Sunday’s semi-final to reach her first global outdoor final.

“I’ve been in the semi-final once before but my aim at this championship is to improve on last year,” she said. “There’s no point getting ahead of yourself. I am where I am, I made the semis so that’s great. I’ll try qualify, run fast and see what happens.”

There was no such joy for Laura Nicholson, who was left trailing in her 1500m heat when the gears shifted on the penultimate lap, the Bandon athlete coming home 13th in 4:14.12.

“It was a bit rough, to be honest,” said Nicholson. “I didn’t feel the best, it’s my first major track champs and I was a bit starstruck with the lights and everything, but it was a good experience. This season, I’ve made a huge jump forward but I’ve also been humbled a lot – I see the ways I have to go.”

On Sunday, Fionnuala McCormack will be the first of the Irish athletes in action, the 40-year-old mother of three facing hot, humid conditions in the women’s marathon 18 years on from her World Championships debut in Osaka.

Andrew Coscoran and Cathal Doyle will be in the men’s 1500m heats that follow, with both hoping to follow the lead of Healy and O’Sullivan, while Nicola Tuthill will hope to challenge her Irish U-23 record in the women’s hammer qualification round.

Sarah Lavin will race the 100m hurdles heats at 3:05am Irish time, where a top-three finish will be needed to automatically advance, while later in the day Mawdsley and Becker will race the women’s 400m heats.

Healy and O’Sullivan will also be back on track for the 1500m semi-finals, while Efrem Gidey will race a straight final in the 10,000m.

World Athletics Championships – Live, Sunday: RTÉ Two, 10.30am; BBC Two, 12:05am/11am

Irish in action, Sunday (all times Irish)

11.30pm (Saturday): Fionnuala McCormack – Women’s marathon

1am: Nicola Tuthill – Women’s hammer throw qualification

1.35am: Andrew Coscoran, Cathal Doyle – Men’s 1500m heats

3.28am: Sarah Lavin – Women’s 100m hurdles heats

11.25am: Sharlene Mawdsley, Sophie Becker – Women’s 400m heats

1.05pm: Sarah Healy, Sophie O’Sullivan – Women’s 1500m semi-finals

1.30pm: Efrem Gidey – Men’s 10,000m final