Van der Sar, a Dutch goalkeeper, recalled the confident forward, who was already demonstrating a strong work ethic, saying he would one day become the best player in the world.
Ronaldo has since achieved incredible success, scoring nearly 950 goals at the highest level and winning five Ballon d'Or awards, as well as multiple Champions League titles.
In a recent interview, Van der Sar shared the words Ronaldo told him early in his career.
"Cristiano was incredibly dedicated; he was always training hard and pushing himself. He would tell me, 'Edwin, I'm going to be the best in the world.' I advised him not to set his sights too high, but he proved me wrong," Van der Sar said.
Ronaldo and his team, Al-Nassr, will return to action on Wednesday, starting their AFC Cup campaign against Istiklol from Tajikistan.
