As a player, Michael Chang was the most successful Asian-American in the game, winning the French Open as a 17-year-old in 1989 and reaching No. 2 in the world rankings. As a coach, he's helping Asian tennis hit new heights, working with Kei Nishikori, who became the first Asian man to reach a Grand Slam final with his run at last year's US Open, then became the first Asian to rank in the top five on the ATP Tour.

Nishikori, now 25, had already enjoyed a significant American influence in his development as a player, moving to the IMG academy in Florida as a teenager to pursue his tennis career. Although the coaching and instruction helped his progress, the talented youngster found the adjustment to a new country difficult, often unsure of how to balance the cultural contrasts between his old and new homes. It didn't stop him from breaking through at a young age -- he won his first title at 18 -- but frequent injuries interrupted Nishikori's climb, and he found himself stuck below the top 10 before turning to Chang in the 2013 offseason.

Nishikori hugs Chang in the audience after his Rakuten Open win in Tokyo last year. Their relationship has grown along with Nishikori's game. Koji Watanabe/Getty Images

Though Chang had previously done some coaching for the Chinese federation and helped a few players, he wasn't looking to go back on tour when the position came up. But in Nishikori, he saw someone who played the way he did, making up for a lack of power by being an excellent mover. He saw someone who was having the same problems he'd had once he reached the top of the game. And he saw someone with a personality a little like his.

He wanted to help, and something told him he could.

"I think there are similarities there," Chang said during Nishikori's run at the US Open. "There are style similarities. Obviously being Chinese, he's Japanese, there's cultural differences, but there's cultural similarities as well.

"I think mentality, generally, being a little bit more on the quieter side, is something I understand a little more. So those similarities obviously make it a little bit easier to work with him and to communicate."

There was also geographic significance. Chang, an American-born child of Chinese parents, had always been conscious of his Asian roots, aware that his impact as a player had been greater there than in his own nation. Despite his influence, few top players were emerging from the region, and coaching Nishikori was a way to change that.

"I felt I have an opportunity to be able to help a young, very talented Asian player be able to take his game to the next level," said Chang.

Though Nishikori has a large team and has also kept working with his full-time coach Dante Bottini, he credits Chang for important tactical and mental adjustments that have helped him move to the forefront of the new group of challengers in the men's game.

On some level, Chang was coaching Nishikori even before he became his coach, starting with an exhibition the two played for charity a few years ago in Tokyo. They spoke, with Nishikori mentioning how much he admired Roger Federer. That's great, said Chang, then asked the whether Nishikori realized he was still competing with the legendary champion from Switzerland. Nishikori didn't understand. If he went on court with that attitude when playing Federer, explained Chang, he wasn't going to win.

That conversation helped Nishikori start to put aside some of the deference he showed when playing the best players.

"Yeah, I was struggling with that actually, because when I was junior, I wasn't thinking too much, so I was able to play good tennis with anyone. But after turning pro, I was feeling a lot of respect to everybody actually, especially top players," Nishikori reflected at the ATP Tour Finals, where he found himself playing among the top eight for the first time. "Like first time I played [Federer], I couldn't play anything because I respect too much. I wasn't go[ing] for [a] win actually. I was just, you know, play tennis against my idol. That was one of the problems I had."

Though the Japanese baseliner is otherwise a great competitor with an exceptional deciding-set record, he has also benefited from a slightly altered approach to the way he plays. Chang, who was a player known for his heart, describes Nishikori as calmer and more relaxed than he is. He liked that Nishikori is a hard worker, but he pushed him to get more fired up and play a more assertive game.

In their first big tournament together, Chang told his player to step in and hit bigger against Rafael Nadal in the fourth round of the Australian Open. Doing just that, Nishikori found that he could stay on court with the awe-provoking Spaniard, and emerged a changed player despite a tight three-set defeat.

"I was playing different tennis, I think, from that match against Rafa in Australia, step in more," he said, a few tournaments following that tussle. "And, yeah, the game changed my tennis."

Just before the US Open, Nishikori was considering not playing because of a foot injury, but Chang encouraged him to compete, telling him of times he or others had almost not played and then done well at a tournament.

Chang relates to Nishikori not just because of some cultural similarities. They also share the same workmanlike style of play. Toru Yamanaka/AFP/Getty Images

"Get through the first two matches and anything can happen," said Chang.

Nishikori not only won his first two matches, but he reached the fourth round, where his opponent was Milos Raonic, who had defeated a frustrated Nishikori in their previous Grand Slam meeting with a clutch of aces.

"Don't get frustrated," Chang said this time. Nishikori didn't, going five sets and finally capturing the win at 2:26 a.m., after 4 hours, 19 minutes. He went five sets again in his next match, against Stan Wawrinka.

Chang praised the effort and had another message as well. "But [Chang] also says, you know, it's not done," said Nishikori.

"It's easy to be able to be satisfied with that," Chang said of keeping Nishikori aiming for more. "You have the opportunity to win a Grand Slam, you have the opportunity to win your first major, you take advantage of it, and you come as best as possibly prepared as you can."

Nishikori followed up his five-setters with a defeat of No. 1 Novak Djokovic in the next round, and that convinced him he was a Grand Slam contender.

"That gave me a lot of confidence, especially physically," he said heading into the French Open, saying that he previously did not know if he could go seven rounds. "I didn't really see how I could play two weeks."

Nishikori has repeatedly pointed to Chang as helping him acquire more of a champion's mentality, saying, "I learn a lot of things from him -- tennis and also mentally too. Yeah, I love how he thinks. Sometimes it's different, like we think different. When we talk to each other, I try to understand how he thinks."

Having led the way in Asian-American tennis, Chang is now helping take Asian men's tennis forward.