The 22-year-old, better known for her world-class 800m performances, stepped down in distance and dominated her heat. Werro finished well clear of the rest of the field, posting the fastest qualifying time of the day by a significant margin.
The 50.80 performance improved her previous personal best of 51.03, set in 2025, and marked her first time under the 51-second barrier.
Werro arrives in Zurich in outstanding form after a sensational 800m season that includes a national record and world-leading 1:53.80 in Paris. The 400m outing served as a sharp speed session ahead of her 800m races at the national championships.
She remains the clear favourite for the 400m final and will also contest the 800m as she continues her preparations for the European Championships in Birmingham.