I heard a story about Bill Tilden, who used to start a game holding 5 balls. He'd hit 4 first-serve aces, and then spike the 5th ball.
hmm dont think this is a good idea, might trip if u have to run back for a lob or somethingI wasn't alive back in the 60s. So I don't do this. I actually don't like to keep any in my pocket either cause it disrupts my rhythm. I like to keep it on the ground a few feet behind the baseline in a neat little pile.
I started out with a two-hander (was a converted baseball player...throw right, bat left) and kept the second ball in my pocket but I didn't wear regulation tennis shorts back then. Tennis shorts weren't designed with decent sized pockets back then and were too tight.I guess none of those old timers used a two-handed backhand.
I don't think that was a good habit, it's a good thing nobody does it anymore