The rivalry between Novak Djokovic and Roger Federer that captured the attention of the tennis world in second half of 2014 may trail off in 2015, former tennis star Michael Chang says.

That is, if Chang's protégé, Kei Nishikori, has the kind of year Chang is hoping for.

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Tennis.com reported that Chang believes his 24-year-old pupil, who burst onto the scene in 2014 with four wins, an appearance in the U.S. Open finals and a year-end ranking of No. 5 in the world, is capable of winning any of the Grand Slam events in 2015.

"The opportunity for him to win a major is going to be there for him at all four Slams," Chang, Nishikori's coach, told Tennis Channel.

"There is no question about it. As far as going out there and saying we are going to win one, I don't want to be in a situation where I am talking about Kei winning a Slam. I would rather continue taking things step by step and match by match rather than looking it as a whole. Sometimes when you look at it as a whole you get absorbed in that and only focus on that. I don't want that to happen to us or to him. It is in the back of our minds but it has to happen point by point and that is the proper mentality."

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Chang adds the health is a key component of Nishikori's future success. Nishikori labored with a leg injury in the second half of 2014, and a back injury forced him to withdraw in the final of the Madrid Open, in which he had Rafael Nadal on the ropes and poised to win the best-of-three match in straight sets.

"If I have one goal for next year that would be for Kei to go out there and play 95 percent of the year healthy. To go through the year without having an injury or something happen that takes you out for two to three weeks at a time would be a huge accomplishment and that alone would take his tennis to the next level."

Of course, No. 1 Djokovic, No. 2 Federer and No. 3 Nadal may have something to say about Nishikori's potential stardom.

Djokovic has stated his goal is to break through at the French Open to complete his career grand slam.

Federer told Perth Now that his fitness level is a big reason for his bounce-back 2014 and that he intends to play as many tournaments as possible in 2015.

"People are always going to tackle me down, 'He is old and not so fit anymore,' but I feel I have never cramped on court," Federer said. "I am one of the fittest guys on tour, I back myself (to win)."

Nadal also is trying to overcome several injuries that slowed him in 2014, but he is the owner of 14 Grand Slams, second to Federer's 17, and he has won nine of the past 10 French Opens.