British heptathlete is in a medal position at the end of the opening day in Eugene, Oregon.

Thea Brown led the world under-20 heptathlon after the opening two events and finished day one in bronze position, while fellow Brit Wyatt Larkins smashed his hammer PB to qualify third for Friday's final — before the USA stole the global headlines with a mixed 4x400m world record in the evening session at Hayward Field.

Brown, of Sale Harriers, who arrived in Eugene having finished runner-up to Finland's Enni Virjonen at the 2024 European U18 Championships, opened with a personal best in the 100m hurdles before adding to her tally in the high jump to briefly hold the overall lead.

She could not quite maintain that position through the remainder of the day, but there was still plenty for the travelling support to celebrate: a personal best of 11.85m in the shot put was followed by 24.45 in the 200m as Brown closed out day one in a close third place on 3632 points.

It leaves the 19-year-old in touch with the medals heading into Thursday's long jump, javelin and 800m, with Virjonen setting the pace on 3676 points, just ahead of France's Zola Ndouma-Mona on 3669 after the Frenchwoman's championship heptathlon best of 14.72m in the shot put.

There was another standout British performance in the field, where Larkins produced the throw of his life in the men's hammer qualification. The Great Britain and Northern Ireland thrower smashed his previous best with 73.10m to qualify third for Friday's final.

The evening session, though, belonged to the hosts. The United States rocked Hayward Field with the fastest under-20 mixed 4x400m relay in history, as Joshua Shelton, Taylor-Nicole Overton, Jaelen Hunter and Olivia Harris combined to stop the clock at 3:15.31 — more than two seconds inside the previous world best of 3:17.69, set by another American quartet at the 2022 championships in Cali.

Czechia's Linda Botková, fresh from her 400m hurdles gold at last month's European U18 Championships in Rieti, hauled her team from sixth to second with the fastest anchor leg of the race, crossing in a national U20 record of 3:18.16, with Australia taking bronze in an area under-20 record of 3:18.45 as Jamaica also dipped under the old global mark without reaching the podium.

Elsewhere on the track, Kenya's Frankline Kibet added the world under-20 5000m title to the world under-20 cross-country gold he won in Tallahassee in January, holding off team-mate Nehemiah Kipngeno (13:30.09) as Ethiopia's Damitew Kebede produced the shock of the distance events, slicing almost 20 seconds from his personal best to snatch bronze in 13:35.38.

In the corresponding women's race, Uganda's Charity Cherop timed her celebrations to perfection, crossing the line in 15:03.88 for gold ahead of Ethiopia's Shito Gumi (15:07.91) and Kenya's Joyline Chepkemoi, who also took almost half a minute off her own best to claim bronze in 15:09.08.

Brazil's Alessandro Borges provided the moment of the night in the field, launching a personal best of 20.35m in the final round of the men's shot put to snatch gold from Korea's Park Si-hoon, who had led with 20.31m before Borges' late intervention. South Africa's 16-year-old Jaco van Dyk announced himself with a fifth-round PB of 19.80m to claim bronze in a stacked competition.

The sprint events also began to take shape, with USA's Tate Taylor clocking the fastest time in the men's 100m semi-finals in 10.06 ahead of South Africa's Marko Ferreira (10.13), while world under-20 leader Gary Card of Jamaica progressed more cautiously in 10.16.

In the women's race, USA's Mia Maxwell led the way in 11.13 ahead of twin sister Mariah's 11.25, with Jamaica's Theianna-Lee Terrelonge and world leader Shanoya Douglas both clocking 11.21 and Switzerland's Xenia Buri setting a national U20 record of 11.23 to join them in Thursday's final.

Italy's 16-year-old Kelly Doualla, fresh off her sensational European U18 100m gold in Rieti last month, also safely negotiated her heat in 11.36 — bringing an eventful opening day at Hayward Field to a close.