I run regularly and the rule of thumb is 100 calories per mile so 600-700 calories per hour depending on speed for me. Now running is an aerobic activity and you get into a groove and if you have enough water and are getting enough air, then your body should convert fat to sugars for fuel.
Tennis is more of an interval-type activity and interval-type activities generally keep your metabolism elevated for some time after the actual activity so you'll get some additional calorie burn there. My personal feeling is that tennis is a lot more intense than running is and that you can burn more calories if you are really moving, hitting your serve well and are playing someone that really works you. If you're hitting casually or just practicing or taking long breaks or taking calories on breaks, then your net calorie loss is going to be lower.
I don't focus on calories for tennis but I do focus on the input side and pay some attention to the output side, particularly for running where it's easier to measure. If you want to lose weight, start out with the MyPlate application at the livestrong site and track what you eat so that you can see what's coming in and then maybe make some decisions on intake. I find that it is a lot easier to control your calories to lose weight than it is to try to exercise it off.
4 x IG Prestige MP, 70 cm, 376 grams, 386 SW, ALU Power @54, Prince Syn Gut 16 @58