Burrow nailed it basically.
I read that Connors liked to eat a big meal a good 2 hours before a match, allowing for digestion, and when he hits the court, he is peaking.
Nastase liked to be "on the verge of hunger" before taking the court, according to his book.
Then you have someone like Rosewall who liked to eat no more than 1 hour before his match, surely he is an ectomorph. Then you have Laver or Tilden who would have no problem scarfing down a big meal 30 mins or less before a match
Tilden even said, of eating steak and potatoes just before a match: "Better to be full and sluggish at the start of the match and have energy for the duration, than to come too empty, play well at the start, but wilt during the peak of the match."