On 17 May 2015, Williams scored his first goal in the top flight, netting a last-minute winner in a 3–2 away victory over Elche to help Athletic come from behind 0–2. Thirteen days later, he headed a consolation goal in the 3–1 loss to Barcelona in the final of the Copa del Rey at the Camp Nou. He missed the 2015 Supercopa de España victory over the same opponents three months later through injury.