Shuzo Matsuoka said on Tuesday that Nishikori is already technically better than world number one Novak Djokovic after breaking into the top five for the first time in his career last year.
"Kei is hitting the ball harder and controlling opponents more than last year," Matsuoka said in an interview.
"He has a decent chance of winning the French Open. He's knocking on the door, for certain. I'm not being biased but he plays better tennis than Djokovic or (Rafa) Nadal."
"That said, he knows there are still questions over his mental strength and that tennis is about more than just pure talent.
"Seeing Kei dominating on clay puts him right behind Djokovic in terms of who you would fancy to win right now. I don't think anyone would write off his chances at the French Open. He has a good shot."
The 25-year-old Nishikori defended his Barcelona Open title last week to win his ninth professional title and Matsuoka believes he is a far more mature player than the one that was humiliated by Marin Cilic in last year's US Open final.
"He had never reached a grand slam final before and he felt the pressure for sure," Matsuoka added.
"But it's only a matter of time before Kei wins a Grand Slam. If he wins one, he could go on to win a bunch."
Matsuoka, who became the first Japanese player to win an ATP title in 1992, also believes his former pupil has little to fear from the so-called big four of Djokovic, Nadal, Roger Federer and Andy Murray.
"Murray had it incredibly tough. He was coming up against the other three at their absolute peak. But their powers are fading a little now, without a doubt, and a new generation of players are on the rise," Matsuoka continued.
"Clearly, Kei can beat those players and this year he has a bigger chance of winning a slam than last. In the next two or three years, as long as he stays fit, he will break through."
Nishikori first burst onto the scene in 2008 when he won at Delray Beach as a 244th-ranked qualifier, but since then he has developed a huge following in Japan.
However, throughout his career he has been struggling with injuries and needed major elbow surgery in 2009.
"He's tougher now. And he can also play mind games if he has too. He is very adept at reading his opponents and out-psyching them. He can lull you into thinking he's down and out – and then hit back and knock you out in the fifth set," Matsuoka said.
"For me there are two true geniuses in the game – Federer and Kei. Their touch, their creativity. Kei still has to win a grand slam but he is playing super tennis and can smash that wall down."