Billie Jean King calls Novak Djokovic "unique" and "smart", says the Serb is "more forthright about change" than Nadal and Federer

Billie Jean King believes Novak Djokovic is more outspoken about matters of change in tennis than his 'Big 3' rivals Rafael Nadal and Roger Federer. The American great also gave her assessment on whether a rivalry similar to that of the legendary men's trio would be good for women's tennis.

Djokovic's founding of the Professional Tennis Players Association (PTPA) is an example of him taking an active role in trying to improve the sport. The Serbian launched the group along with Vasek Pospisil last August during the US Open.

The association represents singles players in the top 500, and doubles players in the top 200. Djokovic formed the group with the intention of helping lower-ranked players by ensuring fairer distribution of prize money among other things.

In a recent interview with AS, King, who fought for social change and equality during her career, praised Djokovic for his contributions to the sport.

"Djokovic is unique and different," the 39-time Grand Slam champion said. "He's very smart and more forthright about change than the other two.

"I wish they weren't so sure, that they talked more off the track, because it has to be done. Federer came out once for the players' association to stick together, and he was great, but there is still a lot of work ahead."

A Big 3 like Novak Djokovic, Rafael Nadal, and Roger Federer would be good for women's tennis: Billie Jean King

Novak Djokovic, Rafael Nadal, and Roger Federer at the ATP Heritage Celebration in 2013

Billie Jean King also weighed in on whether competition between a select group of players similar to Novak Djokovic, Rafael Nadal, and Roger Federer would be good for the women's game.

The American great highlighted the era of Martina Navratilova and Chris Evert as a period similar to the current golden generation in men's tennis.

"Before, when there was a rivalry, like Chris and Martina's, it was generally said that there was no depth in the women's circuit," King said. "There is always this discussion in the media. Now there is much more depth, many more good players, and it is said that a rivalry is missing. It's like always wanting what you don't have."

Despite pointing to there being greater depth on the WTA tour at present, King suggested a great rivalry could take women's tennis to the next level.

"It would be ideal to have it all, some rivalries, not just one, more than one," King continued. "But in women's sports and in tennis in particular we have much more depth now because there are more people who have paid attention to us. The other is always great, but do we need it? Probably. In any case, I think it would be nice to have both."