[I'm writing this having watched the semi-final of Tsonga def. Nadal, but don't know how Tsonga will do in the final.]
Tsonga's win over Nadal illustrates perfectly how less effort can translate into more results in tennis. Knowing that Nadal likes to grind his opponents down, Tsonga refused to get into a competition of emoting and fist-pumping, which would just play to Nadal's strong point of endurance. Instead, Tsonga played it cool between points and after winning games. Plenty of effort to race after defensive shots or bash his own winners, but dialling down the energy afterwards. No elaborate jumping around while waiting to return serve. And when faced with favourable court geometry, winning points with short dinks and drop-volleys. (Amazing how many players have no strategy for approach shots or volleys other than hitting risky deep shots, that are easy for a fast player to get if they move early and guess right.)
I was also impressed at Tsonga's crafty approaches, again taking a cue from Fred Thompson by coming in later than anybody else would. Nadal has his options worked out in advance when someone hits an obvious approach shot and rushes in behind it. Tsonga paused just long enough for Rafa to decide on a safe high defensive shot, then closed for winning volley.
Tennis is like rock climbing in that the best strategies often require less physical effort than you would think. In rock climbing, you keep reminding yourself to just hang there with muscles relaxed when you're not actively pushing higher. In tennis, you should remind yourself periodically that the racquet and the ball each just weigh a couple of ounces, you should never feel like you're straining to swing hard.
To get a power boost on groundstrokes, start swinging a little before you normally would, accelerate gradually but steadily, and keep the followthrough going until the swing stops by itself. It's when you tense the opposing muscles, either in anticipation of a sudden impact or to stop the swing suddenly, that each stroke feels like a major effort. Keep things loose, and your opponent's hardest shot feels like a feather coming off your strings.
For a faster serve, visualize yourself using 75% effort. Making yourself relax a little can (paradoxically) produce more power than straining trying to fire away at 100%.
I also like to minimize the exertion while waiting to receive serve. Some amount of rocking, flexing the knees, or going up on the toes can keep you ready to move in any direction, but the motion should always be slight enough to feel effortless. Otherwise, you'll tire yourself out in long matches or lengthy receiving games.
One last tip along these lines. You've probably heard advice to support the racquet with your opposite hand. That's not just to keep everything lined up at a sensible angle. Use the opposite hand to take the weight of the racquet between points; even drop your racquet hand occasionally and support thcquet at the throat with your other hand. That can make a big difference in whose shoulder or wrist cramps up in the third set!