Welcome and thanks for posting your query to the forum.
It seems from your problem that you have "Purpuric spot". This is a common phenomenon after any minor frictional injury
over skin. In this condition the blood collects underneath the skin. Initially it appears as reddish and then turns purplish in next few days. The complete normal skin status takes few weeks of period.
In your case as you are a tennis player so during playing you may get trivial frictional trauma leading to purpuric spot. You need not to be worried about this spot if it is not progressing in size. No active intervention is required for the same.
If the spot is increasing or multiple new spots are forming then you you must consult either a physician or a dermatologist
Meanwhile eat a lot of Vitamin C
Hope these informations are helpful.
"Wish you good health"
Dr Sanjay Kumar Kanodia
MD (Dermatology & STDs)