Díaz changed his focus to the 1500 metres from 1997 onwards and he knocked more than six seconds off his personal best that year, setting a time of 3:34.52 minutes. He reached the 1500 m final at the 1997 IAAF World Indoor Championships and finished in fifth place. A consecutive fourth-place finish at the 1998 European Athletics Indoor Championships saw him again outside of the medals on the major stage. He improved his time further to 3:32.17 minutes in July 1998 and came seventh at the 1998 IAAF Grand Prix Final that year. A surprise European indoor record for the 1500 m came at a meeting in Piraeus in February 1999 as he bettered Peter Elliott's record by over second with a run of 3:33.32 minutes. He ran another fast time at the 1999 IAAF World Indoor Championships the following month, just a little slower than Elliott's former record, but was beaten by Haile Gebrselassie and Laban Rotich, taking the bronze medal for his first podium finish on the major stage. The 1999 World Championships in Athletics in Seville saw Díaz, alongside Reyes Estévez and Fermín Cacho, vying for the 1500 m medals on home turf. In the fastest race ever seen at the competition, Díaz ran a personal best of 3:31.83 minutes but he ended up behind his compatriots with a fifth-place finish.