Sydney McLaughlin-Levrone smashed the long-standing United States women's 400 metre record to surge into the final at the World Athletics Championships on Tuesday.

The 26-year-old American, who is the world record holder at the 400m hurdles but has chosen to run the flat race in Tokyo, blazed through her semi-final in 48.29sec.

That was almost half a second faster than Sanya Richards-Ross's previous US record of 48.70, which had stood since 2006.

McLaughlin-Levrone will be the red-hot favourite to capture her first 400m world title in Thursday's final and said she was "honoured" to better Richards-Ross's national mark.

"Definitely wasn't expecting that time but it just shows the fitness is there," she said.

"Excited for the finals and grateful to have taken down a record by an amazing

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