I shot these with a Sigma 120-300 f2.8 lens, most with a 1.4 teleconverter attached on a Canon 1D MK II N. I generally stopped down a little to get better depth of field and to recover some sharpness lost because of the teleconverter. I also picked angles where the background wouldn't be an issue even though I was shooting stopped down. I tried to keep the shutter speed up above 1/4000, and at times was up at 1/8000. This meant that my ISO was anywhere from 200 to 400 for most. I think I might have gone higher for some too.
Your lens should be plenty sharp enough - I really doubt it is the issue. Just watch your shutter speeds and make sure they are high enough that camara shake doesn't become an issue. That is harder in Tennis since you are constantly moving with the player. Just try to find something to brace yourself with. I used to use a monopod, now I just get creative to find things to rest my elbows or lens on. That in itself will improve your shots. It did for me.
The last bit is my camera is set very neutral, and I shot these all in RAW. When I process them, I do no sharpening in the RAW converter, but in Photoshop Elements sharpen the images after they have been reduced for screen presentation. Much of the difference you might be seeing could be attributed to that as well. :astly, making sure your images have good contrast lends to sharper looking images as well. The blond young lady was a challenge as she had fare skin and blond presenting a very neutral subject -and some shots just didn't look very sharp that were actually tac-on in focus. Many things impact the look. Just keep plunking away at it. It comes over time. Cheers!
heh
