Today a student of mine was bragging about how hard he works off the court. He told me that he has been running a few miles a day. Problem is..and he has heard this before, running long-distance is no good for tennis unless you are trying to lose weight or master the 7-minute mile stride that is never used for tennis. Plus, it takes time to run long-distance and he as most high school kids are looking to save time with better results rather than take more time to get worse results. The best way to gain stamina is to make your off-court training more tennis-specific in terms of a work-rest ratio. Do some kind of all-out activity (sprinting, back-peddling, etc.) for 20 seconds and then rest for 20 seconds. This will get you tired very quickly and help improve your fast-twitch muscles which you use as a tennis player.
The other mistake that I can think of regarding training is how some kids like to solely work their "show-off" muscles hard. Push-ups, bench pressing, chin-ups are great for getting dates but not so good for winning matches. Nadal may have some guns but more importantly, he has an equally strong core and extremely strong legs. Rather than max out at 6 or 8, max out at 12 for upper-body and 15 for lower body. Most experts agree that one's core is the most important for everything else. You could do even more repetitions of these to get that six-pack and improve your game.
Ricky Becker is the Director of Tennis at Pine Hollow Country Club. He independently coaches high-performance juniors and adults of all-levels year-round at Bethpage Park Tennis Center. He has coached hundreds of ranked junior players. As a player, Becker was awarded Most Valuable Player for the 1996 NCAA Championship Stanford Tennis Team and 1989-1992 Roslyn High School Tennis Teams, and was ranked number four in the United States in the 18 & Under Division.