In an interview with the New York Times Na said Williams, Maria Sharapova and Petra Kvitova are the players to watch on the women's tour.
The defending Australian Open champion said: "Serena is always tough. As long as she wants to be at the top, she will be, because she is such a good player with a mind of a champion. I think Maria Sharapova is going to have a great year. She is just so committed to the sport and will always give herself a chance. Petra Kvitova is definitely going to have a big year, she really worked on her fitness and has gotten so much quicker on the court. She is always the player to beat."
The Chinese star also spoke about the up and coming talents on the tour.
She added: "It is nice to see younger girls playing well like Madison Keys and Garbine Muguruza. I see both of them as future Grand Slam champions. Madison is so talented and has a big future. There is a whole new generation of young players who are so talented and that is so great for fans to see—definitely an exciting time for women’s tennis."