Djokovic doesnt get hit in the face&body while playing.
Try training some martial art, no matter how good of a cardio you think you have (either from running, cycling or some other bullshit) you will be gasping for air after 3 minutes of rolling / sparring.
You learn pretty soon that there are different types of cardio and there are massive differences between them. You think you have a good cardio cause you can cycle for like 100km in one run? Yeah, that wont mean you have enough cardio to run 20km - same goes for martial arts, if you have good cardio cause of BJJ, wont mean you will have good cardio at say muay thai, your legs will give out rather soon after several kicks.