Hannah Brier finished sixth in the 100 metre final at the IAAF World Youth Championships in Cali, Colombia.
The 17-year-old Swansea Harrier had run a Welsh record equalling 11.39 seconds in the heats and 11.55sec in the semis.
But Brier could not reproduce that form as she ran 11.66sec in the final, won by USA's Candace Hill in 11.08, with Khalifa St Fort second in 11.19 and Jayla Kirkland third in 11.41.
Brier is the Welsh 200m champion but is not competing in that distance in Cali.
There was also a near miss for Emily Wright in the long jump.
The Welsh youngster, 17, was in the bronze medal position with a lifetime best of 6.21 metres until the final round, when Slovenia's Maja Bedrac jumped 6.22m to snatch the medal.