I am 34 years old and have played competitive tennis for over fifteen years. Over the last several months my left leg has, for lack of a better word, become "weaker" and or not as strong as it once was. Whenever I attempt to take off and place my weight on the left leg it feels as if it could give out beneath me. I also have noticed this on my serve whenever I attempt to lean into the court onto my left leg. There's no real pain, just a sense that my left leg is not as stable as it once was. Initially I thought the problem might be with my left knee but now I'm not so sure. Does anyone have any insight as to what the problem might be? More importantly, does anyone have any idea as to how I might remedy this ailment?