I agree with Phil, the flexibility Sampras had or has is very incredible. Guga also has "freaky" flexibility. Kind of like Exorcist flex.
Some people just dont have that kind of flex. Strength and flexibility in tennis is a winning combination. As I get older, I notice I am not as flexible especially on the serve, it has really had a small negative effect in some parts of my game.
Former USPTA Teaching Professional
Volkl Tour 10 V-Engine Mid/Luxilon Big Banger