In 2013, the Japanese youngster added Michael Chang to his coaching staff – and with the US Open just around the corner, Nishikori is feeling confident of success

How does one begin to measure toughness? It’s probably most akin to gauging or producing team chemistry, and about as tangible. Lucky for Kei Nishikori and his coach, American legend Michael Chang, both have seemed to arrive for them almost effortlessly, with favorable results nearly as rapidly.

Since naming Chang, the former world No 2 and the 1989 French Open champion, to a support team that includes longtime coach Dante Bottini, Nishikori’s game and results have continued to grow. After turning pro in 2007, the top-ranked Japanese player slowly rose in the rankings up to a career-high No 11 for a handful of weeks in 2013, ending that season at No 17. But talk about impact. Since Chang began working with him, Nishikori, now 25, has won seven of his 10 ATP titles (including three this season), broken into the top five (he’s currently No 4 in the world), and made it to his first grand slam final at last year’s US Open.

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“I think it’s a great matchup with me and Michael, because we play kind of similar tennis, and we have [the] same height,” Nishikori said earlier this month at the Citi Open, which he won. “Mentally, he was really tough. [There’s] so, so many things to learn from him. He has a lot of experience, and for sure, he’s helping my game right now, and my ranking is one of the highest right now.”

Chang didn’t attend the Washington tournament with Nishikori because of the birth of his third child. With Bottini leading the day-to-day training sessions, Nishikori and Chang were still in touch by phone almost every day, though, talking tactics as well as some continued pieces of emphasis for his game.

“Kei is a very reasonable guy who’s probably very willing to listen to Michael,” explains Tennis Channel columnist Steve Flink. “I get the feeling [Michael] is very forceful with him, in a good way, and that Nishikori has great respect for him.” What those finer points are, however, Nishikori – whether for the purposes of gamesmanship or to maintain a real advantage – was keeping mum.

Chang spoke with Flink at the end of 2014, and said his protégé’s serve was a particular aspect he identified early for improvement. “[It] wasn’t much of a weapon,” Chang said. “I looked at some of the stats and saw that the year before, he had served 140 aces and 150 double faults.”

For as much value as specific, concrete adjustments in technique or strategy can make – Chang also discussed the need for Nishikori to play more aggressively, and not just wait for opponents to make mistakes – it’s the more abstract that may have the greater impact. Self-confidence, mental and physical mettle, and perseverance are all quite difficult to quantify, and yet crucial to success. “It is a progression,” said Chang. “It kind of goes with the territory of finding the right mentality. That is something we are working on.”

Facebook Twitter Pinterest Michael Chang looks on as Kei Nishikori plays Novak Djokovic at the US Open last year. Photograph: Julian Finney/Getty Images

Following a quarter-final loss in Montreal to eventual winner Andy Murray, Nishikori had to scrap his appearance at the Cincinnati warm-up ahead of next week’s US Open because of a mild hip injury. But those on hand to watch Nishikori in the nation’s capital earlier in the month could see just how good he can be when he’s playing at his highest level.

Shadows cast deep on the blue-and-green stadium court while Nishikori – consistently said to be one of the game’s best returners – awaited another serve in his distinct open-stance crouch. As his opponent made contact with the ball, Nishikori quickly tracked it and his yellow shoes instantly flashed toward the baseline. He went on the attack like an animal ready to pounce, his racket a sabre dispatched to strike.

Seldom a wasted motion and reliably composed, Nishikori ran through a murderers’ row of elite servers during the week in this manner, among them Australia’s Sam Groth, followed by Marin Cilic, in a rematch of their 2014 US Open final, before toppling American John Isner for the crystal chalice. Throughout the tournament he displayed his prowess for staying the aggressor with his unshakable return game, as well as showed that his own serve was up to the challenge in his fifth visit to the event. It was an impressive display of Nishikori’s blossoming all-around game, even if Chang was not there to witness it.

“Michael has clearly added some mental and physical toughness to Kei’s already formidable skill set,” four-time grand slam winner Jim Courier told the Guardian.

If anyone should know what Chang brings to the court, it’s Courier. The two split their 24-career ATP matches down the middle at 12 apiece. “Kei is playing closer to the baseline on average, which is helping him maximize his fantastic hand skills and play more offense overall. Having someone with Michael’s experience join his team … has been a terrific move.”

Still, maybe the biggest knock on Nishikori has been his health. He’s said he’ll be ready for the US Open despite the hip, but regular injuries throughout his nine-year pro career have taken their toll. So there, too, Chang has provided counsel, with the all-so-important first-hand experience to back it up. When, for instance, Nishikori heavily considered withdrawing from the US Open last year because of the remnants of a minor surgery to remove a cyst on his foot, Chang would hear nothing of it. “And, of course, it proved to be a great decision,” says Flink. “It’s confidence and pushing him, and getting in his face a little bit about it. I think Michael’s probably done that very well.”

Chang’s assertiveness with his pupil hasn’t seemed to be an impediment to the relationship, only strengthening a bond that Nishikori describes as pleasant and easygoing, namely when they spend time together training in California near Chang’s home. “I always stay at his house,” said Nishikori, “and it’s very easy with spending time with him and his family. I’m always enjoying [this] time. He’s a very tough person, especially on the court. But off the court he’s very, very calm and very easy to hang out with.”

How far their unique alliance takes Nishikori – whether that’s to eventually realizing the long-held target of becoming the first Asian man to win a grand slam singles title, or even to the No 1 ranking – is anyone’s guess. For now, the player has only stated publicly a single objective – making it to London, as he did in 2014, for the year-end ATP World Tour Finals. “Ranking is not really my goal,” Nishikori said. “It’s more important that I have to play well, and stay healthy, and try to aim for London this year, too.”

So while measuring toughness, determination and grit remains almost ethereal, should Chang guide Nishikori to where he’d like to end up – no matter how he does it – the only unit actually worth calculating is the number of trophies they together rack up.