He claimed one of only two wins against American grandmaster Hikaru Nakamura at the SA National Junior Chess Championships in Boksburg.
The competition, which lasted more than five hours, was the biggest simultaneous chess exhibition to take place in South Africa.
It saw Nakamura, eighth in the world, take on 101 games at the same time, moving from board to board from 5.50pm until 11.35pm.
Alexander said: “It feels amazing, I have never played someone so highly rated.
“I went wild in the end and played very aggressive(ly) because I was determined to win and he didn’t see what I was planning.”
The Grade 6 pupil said the qualities that made a good chess player were “patience, hard work and passion for the sport”.
Out of the 101 games played, Nakamura won 92 and conceded seven draws and two losses.
He made an average of 24 moves per game and made about 3 000 moves during the course of the simultaneous exhibition.
Khanya Mazibuko, 16, from Soweto was the only other participant to defeat Nakamura.
“I am excited, overwhelmed and very proud of myself.
“I was the second last person to finish; the game was long, but it was fun,” Khanya said.