Stamina is aquired over time, so it's not surprising that he is still rather weak in that regard. He was playing a known grinder after all. It's a bit worrying that his natural ability there seems limited. Otoh Djokovic at that age wasn't any better and now he's ironman, so there is still hope for Andrei I guess. :yeah:
He's by far my favourite of the youngest generation coming up, hope he makes it to the top.