If you have such an item with the pertinent proof of authenticity then it could become very very valuable if he were to win a Major Title simply because he will be the first Greek tennis player to do so. Unless of course Maria beats him to it, in which case he would be the first Greek male tennis player to do so. (Ideally, you should try and get one of Maria's racquets as well. The pair would be worth more than the value of each one individually.)
I wonder what people would have said 20 years ago if you told them that two Greek players would be serious contendors for Major Titles.