American Sydney McLaughlin-Levrone obliterated her own 400 metres hurdles world record with a time of 50.37 seconds to retain her Olympic title on Thursday.

Anna Cockrell made it a U.S. 1-2 in a time of 51.87 and Dutchwoman Femke Bol, who had been expected to challenge for gold, faded in the last 50 metres to take bronze in 52.15.

It was the sixth time McLaughlin-Levrone had broken the world record, lowering her previous mark of 50.65 set at the U.S. trials in June.

The crowd at the Stade de France went into a frenzy when it became clear a new all-time best was within her reach and she fittingly rang the victory bell with a glittering tiara perched on her head.

The first woman to break the 52 and 51-second barriers, McLaughlin-Levrone has been on a mission to rewrite the history books whenever she competes, turning the 400 hurdles into a marquee event - and herself into a superstar - in the process.

"It's amazing to see our sport continue to grow, for people to want to watch the 400m hurdles, it's amazing. Just a lot of hard work put in this year," she told reporters.

"I knew it was going to be a tough race. An amazing competition all the way round."

She walked to her blocks like a prize fighter stepping into the ring, expressionless despite the chorus of cheers that greeted her, and wrested the lead from the moment the gun went off, gliding over each barrier with ease.

Bol, the only other woman to have cracked 51 seconds, was widely seen as McLaughlin-Levrone's closet challenger and had a large Dutch cohort in the stands on her side, but she lost steam five days after she delivered gold in the mixed relay.

McLaughlin-Levrone, the 2022 world champion, was miles ahead of the field down the final straight and she charged ahead in a thrilling race against the clock as Cockrell held on to produce a personal best in second.

"I sacrificed a lot, my family has sacrificed a lot," said Cockrell.

"To do this today, when it counts the most, on the biggest of stages, with my whole family here, we're all over there crying our eyes out. I'm truly a talker and I have no words."

It was the fourth time in a row the United States had won the event on another big medal night for the Americans, as Grant Holloway won the men's 110 hurdles.

McLaughlin-Levrone had an extra reason to party, as she turned 25 on Wednesday.

"Grateful to be celebrating my 25th birthday like this. It was yesterday, just a super opportunity, you can't even imagine," she said.

"My whole family's here. We're going to celebrate, we're going to enjoy this time and get ready for the relay too."