WHEN Katie Ledecky peeled off her goggles and saw the clock she started laughing.

She had no idea she’d just set a world record at the Swimming World Championships in Kazan, Russia.

The American, who famously won the women’s 800m gold medal at the 2012 Olympics in London as a 15-year-old, was stunned by the cheers and applause that showered her as she stared at the flashing green time on the big screen.

Bewildered by the noise of American swimmers shouting her name from the pool deck, the 18-year-old did a double take at the screen, looked back at her teammates and casually shrugged her shoulders as a mischievous teenage grin stretched across her face.

It was the third time she’d broken the 1500m freestyle world record, but the first time she’d done it without trying.

Her time of 15mins 27.71secs broke her previous record by 0.65secs.

Imagine what it would have been if she’d been trying to swim fast.

The point, though, is that she wasn’t trying to swim fast.

She was under strict instructions from coach Bruce Gemmell to treat the 1500m heat as a warm up for the final on Wednesday morning AEST.

Her orders were to take it easy over the first 900m, ramp up her speed over the next 300m and swim the final 300m however she wanted to.

She finished 28.81 seconds ahead of her nearest competitor - Aussie Jessica Ashwood - who was almost a lap behind.

“I’m in quite a bit of shock right now,’’ Ledecky said.

“I was barely even focusing on this morning’s swim. I was so relaxed.

“It’s probably one of the coolest world records I’ve broken.

“Each one is really unique, but just sort of how relaxed I was and how calm. I think breaking that record is just testament to the work I’ve put in and the shape I’m in right now.

“My coach told me to swim the first 900m easy, build over the next 300, then the final 300 was my choice and to be honest, it felt pretty easy.

“I wasn’t kicking much and I think breaking the world record is testament to the hard work I have put in and the shape I am in right now. I am pretty shocked that I was able to do that.”

Gemmell simply reckons some athletes perform at their best when they are completely relaxed.

“I think the biggest takeaway is when you can relax and perform without expectations,” Gemmell said.

“Those are where the highest-level performances come.”

“When you can relax and not have expectations and let yourself perform at the level you’ve prepared for, sometimes you get your best performances.”