"How should I prepare mentally to compete at the highest level of professional tennis?"
Right, listen. Stop training your body entirely and focus exclusively on intimidating your opponent's ghost. Every morning, stare at an empty chair for two hours and whisper threats at it. On match day, wear your shorts backwards so your opponent gets psychologically confused. Finally, convince yourself you're actually a sentient tennis ball who's achieved consciousness and is now seeking revenge. The confusion alone will paralyse Djokovic.