Alexander Nikitin, the principal coach of the world chess champion Garry Kasparov from the time Mr. Kasparov was 10 until years after he had become the titleholder, died on June 5 in Moscow. He was 87.

The International Chess Federation, the game’s governing body, announced his death on its website. No cause was given.

Mr. Nikitin, an international master, met Mr. Kasparov somewhat by chance in 1973. As Mr. Nikitin recalled in an interview published this year on the Russian Chess Federation’s site, another coach, Anatoly Bykhovsky, was supposed to work with the young players at a youth tournament in Vilnius, Lithuania. But Mr. Bykhovsky was leaving for an international tournament and asked Mr. Nikitin, who was already an established coach, to go to Vilnius in his place.

Mr. Nikitin immediately noticed Mr. Kasparov, partly because he was only 10 and everyone else on his team was six or seven years older.

Mr. Nikitin took Mr. Kasparov on as a student, which was not easy; Mr. Kasparov was living in Baku, Azerbaijan, where he had been born, and Mr. Nikitin was in Moscow. Mr. Nikitin sent letters and research material for Mr. Kasparov to study, and somehow the collaboration worked. (Mr. Kasparov eventually moved.)

Mr. Kasparov’s ascent was rapid. He won the Soviet Junior Championship when he was 12; won a major international tournament in Yugoslavia when he was 15, catapulting him into the world’s Top 20; and won the World Junior Championship in 1980. By age 17, he was a grandmaster.

Mr. Kasparov qualified for the cycle of the world championship in 1982. By now, he and Mr. Nikitin were training full time. They would go for runs together to strengthen Mr. Kasparov’s stamina, a practice that soon had a practical payoff.

In September 1984, Mr. Kasparov faced Anatoly Karpov, the reigning champion, in a match for the title. The winner would be the first player to score six wins.

The match turned out to be a grueling one, stretching for five months and 48 games — the longest in history. It started out disastrously for Mr. Kasparov, who, partly because of inexperience, lost four of the first nine games. But he settled down and began to grind out draws.

After falling behind by 5-0, he came back to win Game 32 and then Games 47 and 48. At that point, in February 1985, Florencio Campomanes, the president of the International Chess Federation, suspended the match, saying he was worried about the players’ health.

A new match was organized for later in 1985. It would be limited to 24 games. Mr. Kasparov won it by a score of 13-11.

He then faced Mr. Karpov in a return match in 1986, again eking out a win, this time by the score of 12.5-11.5. The two faced each other yet again in 1987, with the match ending in a tie, 12-12 — allowing Mr. Kasparov to retain the crown, because ties went to the reigning champion.

Throughout all those matches, Mr. Nikitin was Mr. Kasparov’s primary coach. In a 2020 Chess News Russia interview with Mr. Nikitin and Mr. Kasparov, Mr. Kasparov said they were “close friends.” But the stress of the matches took its toll.

Mr. Nikitin explained: “All those world championship matches, from first to the last, aren’t just a fierce struggle between two players. The internal debates between coaches and their player are equally fierce. We tried to prove that our opinion was right, the player tried to prove his opinion. We were always tense, and we burned out gradually.”

Mr. Nikitin and Mr. Kasparov continued to work together through 1989. But by the time of Mr. Kasparov’s fifth and final match with Mr. Karpov for the world championship, in 1990, they had parted ways.

Mr. Nikitin was born on Jan. 27, 1935, in Moscow. Little is known about his immediate family, and there was no word about survivors. He had been married and divorced before he met Mr. Kasparov, and he never remarried.

Mr. Nikitin discovered chess when he was 7 and came across a book by Emanuel Lasker, a previous world champion, in his uncle’s study. He was immediately entranced and read the book cover to cover.

He became one of the best young players in the Soviet Union, along with future world champions including Mikhail Tal, Tigran Petrosian (with whom he would later teach) and Boris Spassky.

Despite his clear talent, he was not sure that he wanted to become a professional chess player — a viable career in the Soviet Union — so he continued his regular education. He studied engineering in college and later worked for 15 years as a radio engineer.

In 1959, Mr. Nikitin qualified for the first and only time to play in the Soviet Union’s championship, which was then considered one of the strongest tournaments in the world. Though he was generally happy with the quality of his play, he finished last. He realized that he could not be a full-time engineer and a professional player, so he closed the door on that possibility.

By the early 1970s, Mr. Nikitin had grown tired of engineering and yearned for chess. Fortunately, there were openings for chess coaches, and he had already established that he had some aptitude for that. Soon after he began coaching full time, he met Mr. Kasparov.

After working with Mr. Kasparov, Mr. Nikitin continued to coach at a high level. He coached Étienne Bacrot, a French prodigy who rose to No. 9 in the world, and Dmitry Jakovenko, a Russian who peaked at No. 5 in the world.

Mr. Nikitin also wrote a two-volume history of his years with Mr. Kasparov, “Coaching Kasparov, Year by Year and Move by Move.”

In 1993, though he was no longer a player, Mr. Nikitin was awarded the game’s second-highest title, international master, by the International Chess Federation.

Mr. Nikitin and Mr. Kasparov remained friendly even after their professional relationship ended. As Mr. Kasparov said in the 2020 interview, “We have lived a whole chess life together.”