Can you prove that those several users are tennis experts? Pay attention: the only way you can prove it to people who are not tennis experts is to provide reliable source which shows that they are recognized as tennis experts.
What is a tennis expert? Someone who is really good at playing tennis? Someone who can memorize a lot of tennis statistics? Someone who understands point construction in great detail? Someone who reports on tennis for a living?
None of these things mean a person will be good at making predictions about events that will happen 2 - 3 years from now, where there are a huge number of unknowns. Knowledge about a subject doesn't enable you to see the future
The best evidence of expertise is to look at the predictors' track record.