Welcome to HealthcareMagic. The joint ache could be due to excessive wear and tear of joints, since you have been playing tennis. Also a history of injury in same affected joint/limb, it all the more aggravates the same.It could even be a fracture, minute enough, for eg.. hairline fracture, difficult to appreciate or a ligament/ soft tissue injury
.An infection of that joint, like bursitis or synovitis
. Also confirm it is not any of those arthritis
like Gouty/ Rheumatoid. However, Gout is bit unusual cause for a knee pain
, nevertheless it can be first symptom for some.So get an Xray, complete blood count
, blood uric acid levels and consult an orthopedician. Meanwhile give relative rest to affected part. Take care.