How to start a letter with self introduction to a friend you haven't seen for ages?
你好，我是 Ivan Gerasimenko。
Hello, I am Ivan Gerasimenko.

我们之前在XXX的比赛第中见过一次面， 你还记得吗？
We met once before at the XXX sports competition, do you still remember?

距离上次见面已经很久了，你现在过得怎么样？
It's been a long time, how are you getting on these days?

(PS: Are you from Serbia? We just found out, Chinese people can go to Serbia without a visa.)