This is the tricky part of trying to improve fitness. A player needs to play these types of matches in order to test your limits physically. At the same token, a player does not want to play a lot of long matches, so he or she can maximize his/her chances of winning Slams.
For Sinner, this is the balance he and his team will need to problem solve going forward with his career. He definitely has improved his fitness this year so that’s a great sign! But it will be a continuous working progress throughout his career.