To be frank, I find this question to be odd. Being a form of entertainment and a sport aren't mutually exclusive, things can be both, and I see no reason to believe Tennis doesn't fit both. Tennis is an activity in which people compete agianst each other and therefore a sport, and it also entertains.
I think any type of sport can be considered a form of enertainment even a sport like tennis. While someone like me may not think that tennis is very enertaining people who do like sports and enjoy the sport itself would love to watch it as a form of enertainment making it both.
In general tennis is considered a sport as it should be. With that being said, sports are a form of entertainment. Spectators of football, basketball, baseball, or any sport are doing it for the enjoyment of it. Spectating itself is not a sport. It is a form of entertainment. Tennis is no different.
All sports are a form of entertainment, I don't see why tennis would be looked at differently. It's a sport that requires a lot of endurance as well as finesse, and it certainly has its tense moments, isn't it entertainment by definition? The answer to the question is technically yes, but I don't see anybody arguing the point anyway.
I believe tennis should be regarded as a form of entertainments, just as many other sports are regarded as a form of entertainment. I believe watching sports matches in person are best and that is why tickets to these events sell so well. I don't believe people really deny the entertaining aspects of sports.