PARIS — Seized by leg cramps, 17-year-old Michael Chang realized there was no point in continuing his futility against world No. 1 Ivan Lendl. He had battled back from a two-set deficit to force a champion 12 years older and about a half-foot taller to a fifth set at the 1989 French Open. Now he could barely move.

But the moment Chang took his first halting step toward the chair umpire to announce his retirement, he changed his mind. If he quit then, it would be easier to quit the next time. Worse, it would tell future opponents that he probably would give up if things got tough. On the other hand, if he kept battling, he at least would tell the world, win or lose, that he was a fighter.

Thirty years later, Chang recalls weighing all these scenarios like rapid-fire calculus in those pivotal seconds of his fourth-round match. His decision to keep going still stands as one of the gutsier displays of toughness on a tennis court.

It also serves as a powerful teaching tool in Chang’s current career as coach of Japan’s Kei Nishikori, 29, whom he has helped lift, over their seven-year association, from 17th in the world to as high as No. 4.

“At the end of the day, it doesn’t matter how tall or strong a player is,” Chang explained in an interview. “I played many players in my lifetime who I knew were much better than I am. But if they don’t believe in their game and can’t put it all together, I had a chance. It’s not about hitting shots; it’s understanding how certain shots open up the court.”

Chang’s 4-hour 37-minute victory over Lendl that day was the most improbable of a series of upsets that culminated in history three rounds later, when he toppled Stefan Edberg to become the youngest man to win a Grand Slam event. In doing so, he also snapped a 34-year drought of American men hoisting the French Open trophy.

Nonetheless, Chang moves about the grounds of Roland Garros today largely unremarked, a 47-year-old father of three whom even ardent tennis fans often recognize only after he has walked past. “Wasn’t that Michael Chang?” they murmur.

The spotlight isn’t something Chang seeks, though he gladly accommodates the demands that come with his job and savors the memories that stir each time he returns to Roland Garros. He seems content to blend in, unassumingly assuming his seat at Court Philippe Chatrier on Wednesday in service to the seventh-ranked Nishikori, who played the role of spoiler in turning back Frenchman Jo-Wilfried Tsonga, 4-6, 6-4, 6-4, 6-4, to advance to the third round.

Much like his mentor, Nishikori prevailed after dropping the opening set by playing smart, consistent, high-percentage tennis.

Reared in Southern California, Chang had never played on clay when the U.S. Tennis Association sent him to work with José Higueras, the Spanish former player hired to develop rising American teens, roughly six months before the 1989 French Open.

Higueras saw potential at once in the slight, 5-8 youngster.

“He was the prototype of a player that could be good on clay because he was a very good mover, was a very good competitor and very smart,” Higueras said. “At one point, I told him, ‘Michael, if you work hard, I think you can do really well at the French or maybe even win it in a couple years.’ And he said, ‘Well, why not this year?’ ”

To that end, they worked intensely on the sliding, timing and strategy required to excel on clay — so different from the hard courts Chang knew best.

Chang did copious homework as a young pro, too, logging detailed notes in dime-store spiral notebooks about other players’ tendencies, strengths and weaknesses.

Today, he is still taking copious notes but on Nishikori’s behalf.

Coaching was never in Chang’s career plans, but he was intrigued when approached about the idea by Nishikori’s team in 2012. The two had met the year before, when they played an exhibition in Japan. Chang liked the idea of mentoring a young Asian man.

Though they come from different Asian cultures (Chang is Chinese American; Nishikori, Japanese), Chang said they share a cultural background that makes communication easier. As he explained, because he understands the Asian tendency to be reserved, he doesn’t necessarily wait for Nishikori to ask him questions in their sessions but offers feedback based on what he sees. He is also mindful to emphasize the good things Nishikori does in practice, aware of his charge’s tendency to focus on the one thing he might have done less than perfectly.

Said Nishikori: “He has given me so many great things that helps me to raise my level. Always, he [is] giving me good motivation and little things I am missing.”

Chang’s work dissecting opponents is far easier today, given access to video of virtually every match on the ATP Tour.

The present-day pro’s life on tour is also different. Virtually all top-10 players travel with entourages that include one or two coaches, a physiotherapist, trainer, hitting partner and agent. Chang, as a young pro, traveled with only his coach, agent and mother, who made him chicken noodle soup each morning in an electric rice cooker, flouting hotel rules.

“She actually stuffed towels underneath the door because if you don’t do that, the smell goes out into the hallway,” Chang recalled with a laugh.

Today, Chang is just one part of Nishikori’s entourage, sharing coaching responsibilities with Dante Bottini, who lives near the player in Bradenton, Fla. Chang, who lives in California, spends roughly 25 weeks each year with Nishikori and supplies his scouting notes year-round, via email or the Line messaging app.

Before Nishikori’s first-round French Open match against an unfamiliar opponent, Chang watched video of seven matches of French wild-card Quentin Halys in his Paris hotel room to prepare his charge, who rolled to a straight-sets victory.

“When it comes down to it, it’s how intelligently you play your game,” Chang said. “It’s using your strengths to combat other players’ weaknesses. No matter how big they are, no matter how strong they are, everybody has weaknesses. It’s a matter of going out there and finding it.”