Tennis is a lifetime game as it needs millions repetitions to be successful but I've started to play seriously at 40.
Please find my reminders to optimize and think high performance off the court:

• Don't aim for technical perfection. If it's good enough it's good enough
• Dedicate session to work the serve, return of serve and net game
• Play more training matches or count points to replicate tournament condition
• Do sprint sessions for explosivity instead of running "marathons". You already run on the court
• Use elastic and light weight exercises off the court to target legs, abs, back and shoulders
• Constantly research the game strategy and tactics
• Review your racket setup with a pro. Get two rackets with similar specs (ask matching to your stringer pro) if you play in tournaments. Gut breaks easily under wet and cold condition. Having a spare multifilament is always a good idea
• Eat anti-inflammatory diet and drink clean as much as possible
• Every training must have an objective: only one specific thing until it becomes unconscious competence
• Build and update list of personalized areas to improve
• If different players report the same issue in your game then seriously think about it
• Tennis is one of the most demanding sport not a physical activity