Díaz was an improved runner at the start of 2015 and set personal bests in the 1500 metres (3:40.15) and 3000 metres (7:56.34). He set a new best in the 10K run, defending his title in that section of the Santiago Marathon with a time of 28:45 minutes. At the age of twenty-one, Díaz established himself as one of the region's top middle-distance runners at the 2015 South American Championships in Athletics by securing his first senior gold in the 1500 m. – one of two winners for Chile at the meeting, alongside Víctor Aravena.