US superstar Simone Biles continued her spectacular return from a two-year break by claiming her 21st world title with all-around gold at the world gymnastics championships on Friday.

Already a winner of team gold on Wednesday, the American produced a dominant display to score 58.399 points and secure the title ahead of defending champion Rebeca Andrade of Brazil (56.766), with American teammate Shilese Jones taking bronze (56.332).

Biles obtained the best score on three of the four apparatuses, with the uneven bars the exception.

It was the 27th time she has medalled at worlds and the 26-year-old gymnast was teary-eyed as she stood on the podium with her sixth all-around world crown.

"Actually, I had something in my eye today that I couldn't get out, I swear it's true," she told reporters.

"But I was emotional, because 10 years ago I won my first worlds and now we're back here," said Biles of her return to Antwerp where she opened her world tally in 2013, winning four medals, including two golds.

"It means everything to me, after everything I've put in to get back to this place to feel comfortable and confident enough to compete."

With two more days of competition, everything suggests her medal collection will be further enriched since she still has four apparatus finals come.

Biles has made a breathtaking comeback, two years after the Tokyo Olympics marked by her episode of "twisties", a temporary mental block whereby gymnasts lose their sense of where they are in the air.

Arriving in Japan as the big favourite thanks to her four gold medals won five years earlier in Rio de Janeiro, Biles broke down and withdrew from four of the five finals.

Back at the highest level, the 1.42m Texan is gearing up for the Paris Olympics after taking advantage of her break to take care of her mental health.

"I was actually less nervous today," said Biles.

"I was so much more nervous for team finals just because that's when everything occurred (in Tokyo) so I was a bit traumatised from that, so today I felt a little bit more relaxed. I'm happy that's over."

'She's like wine, better with age'

"She still has difficulties, she's not perfect," said her coach, Frenchwoman Cecile Landi.

"(But) she proves to everyone that she can be better than before. She is like wine, she gets better with age."

During qualifying last weekend Biles added a new line to her legend by becoming the first woman to complete a Yurchenko double pike vault, which now bears her name as the "Biles II".

On Friday, spectators at Antwerp's Sportpaleis witnessed another extremely rare occurrence when Biles tripped on the mat while preparing for a simple leap towards the end of her floor routine.

The incident, accompanied by murmurs of surprise from the crowd and which drew a smile from the American, came with the gold effectively already secured.

"I just tripped, clumsy," she said.

"I'm like that outside of the gym, so I'm not surprised it happened.

"I felt like my legs were so heavy and I couldn't pick them up."

The podium was entirely composed of black gymnasts for the first time.

"We had a black podium of girls, so I thought that's amazing: 'Black girl magic!'," she smiled.

"Hopefully it just teaches all the young girls out that you can do anything you put your mind to if you train hard."

Biles' 34 world and Olympic medals make her the most decorated gymnast in history.

© Agence France-Presse