I really think Sinner has that mental attitude, he is very level headed, he keeps his
emotions under control and doesnt give much away to his opponent, these traits are
remarkable for a 19 yo.
As for Medvedev, one of my favourite players, he is getting better at controling his emotions.
Having an experienced tennis coach who is able to guide these young players around the
mental skills needed is really important.
Partially agree on Sinner, he is very level headed from what I have seen so far but he is maybe missing that other component I mentioned. Swagger or belief that he can beet anybody and that is also needed when playing against top players. When you check results from Tsitsipas, Medvedev or Thiem even when they were younger they were capable of pushing top players to the limits and winning an odd set or even match against them. I don't see that in Sinner yet.
I am not sure about Medvedev. I would put it differently, he has raised his game and was winning most of the matches so there wasn't a problem with controlling emotions. As soon as he had bad day he broke apart and did not have B plan. He is cool when winning but he needs to work on getting back in the matches when things are not going his way. That is something Djoko and Nadal are doing perfectly and Tsitipas and Thiem also managed to incorporate in their game. In other words amount of matches won after loosing first set.
Sorry for the late response, I totally agree and you know now that you make these points
it becomes clearer, Sinner does need that killer instinct or swagger in his game, that adds
the psychology of the game and we know Tennis is as much about belief as it is about skill.
As for Medvedev, I'm a bit dissapointed in retrospect in his season so far hopefully he will
rise for the French Open.