The best tennis player of all time is a tricky topic. You need to factor in level of competition, number of opportunities and cultural impact -- not to mention the shift in eras. Roger Federer -- who has been talked about as one of the best ever himself -- thinks that he has an answer to the debate. According to Federer, Serena Williams is the best tennis player of all time, and it crosses gender lines.

"It's been fascinating to watch," Federer told Jason Gay of the Wall Street Journal in a deep dive profile of the 20-time Grand Slam winner. "She had a totally different upbringing -- I came up through Switzerland with the federation, she did it with her dad and her sister. It's an amazing story unto itself -- and then she became one of the greatest, if not the greatest tennis player of all time."  

When Gay pushed Federer on whether "greatest tennis player of all time" referred to female tennis or tennis overall, Federer didn't hold back. "Overall," he replied.

On their own, the numbers clearly favor Williams. She has 23 Grand Slam titles, just one shy of Margaret Court for the record of 24. There's a chance she'll be able to catch Court, although she has a tough road ahead of her to do so after her pregnancy kept her out of tennis for over a year. As she comes back, however, there are other factors that come into play when talking about the best tennis players ever.

Racket technology has improved, as have travel standards -- not to mention the simple fact that records weren't considered in the days of Billie Jean King or Rod Laver. "It's not fair to compare, actually," Federer said.

"But we know [Williams] is all the way up there. I'm probably up there with somebody, somehow. Maybe there's a group, a best of five -- and if you're in that group, you should be pleased and happy. Tennis is a funky sport when it comes to that stuff."

Federer is being modest. He's absolutely in the conversation, and since his renaissance last year he looks as unbeatable as ever. Even so, however, the numbers don't lie: Williams stands in a class of her own when it comes to tennis on a surface level. If she catches Court, it will be even harder to argue against her.