TORONTO — In early March, when John Isner was staggering through some of the worst results of his career, Justin Gimelstob invited him to Los Angeles for a much-needed cram session.

Isner had won only two matches in the first two months of the season, yet somehow Gimelstob — the television broadcaster, former professional player and a member of Isner’s three-man coaching staff — was bubbling with confidence.

He consulted David Macpherson and Rene Moller, Isner’s two other coaches, and there was consensus on what was needed. Then, in something akin to a team sport’s mini camp, Gimelstob drilled Isner on specific elements of his game, and over the course of the four days, the coach became very encouraged.

Gimelstob even went so far as to tell Isner and his family that if the player stuck with what his coaches were emphasizing, the results would be superlative.

“I told them, ‘I guarantee you he will have his best year ever and completely turn this around,’” Gimelstob said this month. “I guaranteed it.”

Gimelstob’s bold declaration did not immediately come true. Isner lost the next match he played, against Gaël Monfils in Indian Wells, California.

But three weeks later, 25 days shy of his 33rd birthday, Isner won the Miami Open for his first Masters 1000 title. In May, he reached the fourth round of the French Open on red clay, matching his career best at Roland Garros. Then in July he reached his first major semifinal, at Wimbledon, where he lost to Kevin Anderson in an epic match.

The 6-foot-10-inch Isner and the 6-foot-8 Anderson dueled for 6 hours, 36 minutes until Anderson finally prevailed in a 50-game fifth set. Until then, Isner had reached the quarterfinals of a Grand Slam event only once, at the 2011 U.S. Open.

Isner’s breakout spring and summer earned him a career-high No. 8 ranking. At the U.S. Open, which begins Monday, he is seeded 11th and seeking higher plateaus in his 12th year as a pro: At his home major, Isner has advanced past the fourth round only once.

“I was always a late bloomer,” he said during the Rogers Cup in Toronto earlier this month. “I was always so big, it took me a long time to grow into my body. I always knew that my best tennis was going to be in my late 20s.

“Ten years ago I would have been lying if I said I knew it was going to be at 33. But it’s encouraging to see.”

Isner attributed his recent success to a more relaxed approach on the court and a refusal to fret over the outcome of a match. Whatever happens, happens — and that mindset, he said, has led to clarity of thought and better decisions on court. But the loquacious Gimelstob had a more detailed explanation.

“I am a big believer that John is very unique, so he should be trained uniquely,” Gimelstob said. “There is real work to be done, and John deserves a ton of credit for doing it.”

Isner’s best shot — and one of the best weapons in all of tennis — is his explosive serve, which hammers down from a height of about 10 feet and at speeds that can reach above 140 mph. The struggle has always been to match his ferocious serve with a more comprehensive all-around set of skills, but that is never easy for one of the bigger players on tour.

Thus, the mini camp. In Los Angeles, the task was to “clean up” all aspects of Isner’s game, as Gimelstob phrased it, and to put him through numerous repetitions in several categories. Those included shortening his backswing on certain shots and sharpening his volleying so that he felt more comfortable coming to net. They also focused on footwork, to improve Isner’s balance and his ability hug the baseline and punish an opponent’s second serve. They also worked on his racket-head speed and making a firm commitment to whatever shot he is making.

Isner’s powerful serve keeps him in most battles, and also produces many tight matches in which he and his opponent have trouble solving the other’s serve. Two glaring examples are Isner’s loss to Anderson at Wimbledon, and his 2010 victory over Nicolas Mahut that required three days and 138 games in the fifth set, the longest match in tennis history.

“I believe in the analytics of sports,” Gimelstob said. “Think about the math: What moves the margins in your favor? The margins in tennis are small. But there is no one whose margins are smaller on a match-to-match basis than John. Just watch him. So, most of our conversations are about what moves the margins in his favor. If you play the right way, over the course of time it will pay off with big results.”

Deciphering and applying voices from three coaches could challenge many players, but Isner says he relishes the triangular support, and he finds it refreshing to divide his travel, taking different members of the group to different tournaments. He will often rotate time with each one of the coaches individually, but they remain in contact with one another about the goals and the message.

“I have a very eclectic team,” Isner said with a chuckle. “A lot of different personalities into play. All three of them really get along and they supplement each other really well.”

But there is another voice tugging at Isner now: his own body, voicing the relentless refrain of fatigue. Even as Isner has pocketed several career-best results this year, the grinding endurance test of that Wimbledon semifinal, followed by a scorching summer, could be taking a toll.

After Wimbledon, Isner rested for a few days before entering the ATP tournament in Atlanta, which he won for the fifth time. But then he lost his first match in Washington, fell in the third round at the Rogers Cup in Toronto and dropped his first match at the Western & Southern Open in Mason, Ohio. That last loss, to Sam Querrey, could serve as a blessing as it afforded Isner some much-needed rest heading into the U.S. Open.

But it also followed a pattern from the summer of 2010. After his three-day ordeal against Mahut at Wimbledon that year, Isner reached the Atlanta finals, but went 2-2 in two tournaments before he lost in the third round at the U.S. Open to Mikhail Youzhny.

The summer hardcourt season, in steaming, humid conditions, can be a monthlong slog for Isner, who is known to suffer the heat more than many.

“My theory is that I’m just closer to the sun than everyone else,” he quipped, and added, “I weigh more than everyone else, I’m taller than everyone else. When it’s really humid and hot outside it’s going to take a bigger toll on me.”

To help preserve his body, Isner generally spends less time practicing on court than many of his contemporaries, compensating with off-court conditioning routines like Pilates and SoulCycle. Scheduling rest is also critical, because Isner is bigger than most, and also because he is 33, playing the best tennis of his life.

“Maybe 15 or 20 years ago, getting into your 30s was a death sentence,” he said. “It’s not so much anymore. The focus for me is just like everyone else, to be peaking going into the U.S. Open.”