Rune took on Djokovic a few times and cramped against him, much like Alcaraz. It happened at the US Open, where they played the first time, and Rune actually won a set in that match.
Besides the obvious physical stress from playing tennis, nerves contribute a great deal. When you play against arguably the greatest of all time, the pressure is much different, and it may get to you.
I also cramped the first time I played Novak so I can relate to that. It’s not abnormal. You have massive respect and you have been watching them on TV for a very long time so it feels weird the first time to play them, to actually be on the court with them. But it’s nice because it’s like ‘wow I’m actually getting at that level’ and then you realise that you are not on that level.
For Rune, just playing against Djokovic and then actually beating him did wonders. Alcaraz was similar, but he actually cramped after beating Djokovic, however that was on a really hot day in Paris, so there are some things that made it more likely to happen.
So then you’ve got to go back to do the work and then when you finally start to beat them and start to compete with them more and more close, it feels different.
We'll see what happens this year and which players show up to properly challenge Djokovic regularly. Rune and Alcaraz are certainly very good candidates. Their first chance is coming up shortly in Melbourne.