I'm a tennis beginner. im flat footed and feel little strain above ankle. there is no other associated complaint?

Orthotics: I highly recommend you see a podiatrist who can prescribe and make you custom made Tennis orthotics. These will help you a lot through out your career as a tennis player. Pleas consider them.

Muscle memory.: Since you are a beginner and the strain is above the ankle, I'm not sure that your complaint is not due to the newness of the biomechanics involved in tennis. There are many new maneuvers that that you are learning. Any one of these could be responsible for your strain.

Tendinitis: With flat feet the tendons in the front of the leg are strained on daily walking. The strain is magnified with athletic activity. Make sure you have a good shoe with medial arch support. You may require an orthotic (shoe insert) as well

Arch Supports: Being flat footed can cause strain in the arch and the muscles and tendons from your legs all the way down to your feet. Tennis and other running/walking activities lead to extra strain. Custom Orthotics can be made to realign your foot in neutral position as the picture shows above. Visit your podiatrist who can evaluate your feet and can fit you with your own orthotics.