Can't find them now but Kyrios' numbers for fitness like total distance run exceeded Nadal's. This was across several fitness stats. I will look for them tomorrowRespectfully...I disagree. Fitness has a psychological impact. Players use drop shots more late in matches when they are losing. They are trying to end the point early because they know how hard it is to hit the ball accurately and get to the ball with balance. Fitness makes a huge difference. Djokovic used to fall to bits at the Aussie Open in his early career. Injury time-outs, lying down on the court, sulking. He couldn't handle the conditions because his fitness wasn't there and it affected his mental game. The biggest difference in his game is his fitness level. The sulking on court disappeared when he got serious about it. Federer (who I hate) is extremely fit. Murray's natural game gets him into the top 25 - top 20. The thing that got him into the top 5 was his fitness. His ability to stay in points longer and still be hitting the ball with pace and direction 10 shots into a rally. Nadal is a beast. To play from so deep in the court and put the amount of work on the ball he does - again and again and again - takes a massive engine (and a physique like a hermit crab, that left arm is obscene).
Kyrgios has the game to beat anyone. His best chance would be to look at old Djokovic footage, to understand that getting is body into a better place calmed his mind. That is the path for Nick if he wants it. But "if he wants it" is a huge question. His later 2019 and start to 2020 suggests he seems to want it a bit more lately.
That is why l believe there is another reason other than fitness