– 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 — 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 minicamp, 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 Gael Monfils in Indian Wells, Calif.

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 6-10 Isner and the 6-8 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'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 minicamp. 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 to 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."