I know Dusan Vemic is an old guy and he mostly focuses on doubles, but this is a good interview on our page and it's one funny answer to the question:
Do you have any pre-match rituals?
Rituals are so individual, and even though I don't think I am one of the most superstitious athletes or tennis players, I had a one or two of them: meals that I have on a day of my first match (if I keep winning of course) I have every single day,
I use the same clothes I have played my match (washed, by a tournament or with my own hands, depending on if I have a day off or not), used same racket (i don't break strings during a match, so I restring them after each match that I win), use the same shower in the lockeroom, sit at the same table, warm up on the same court, get up on the same side of the bed,
Nothing too much as you can tell and of course, the most important, rituals during the match are the only once that make a difference in a match (taking the same number of ball, using the towel after each point, bouncing the ball same number of times (zero), placing the racket the same way I do from the start of the tournament), organizing energy drinks and water in the same way on the bench...