Todd Martin is difficult to miss. In Novak Djokovic's box, he's the one who takes notes. Many, many notes. On the walk from Arthur Ashe Stadium to the players’ lounge, he's the one who’s so tall that his silver-streaked hair nearly brushes the concrete beams hanging over the walkways. Among admirers, he's approachable and amenable to group photos. He's fond of deadpan humor.

"Todd, could you sign this?" one gentleman, standing with his wife, asked Martin on Wednesday, after Djokovic advanced to the semifinals with a four-set victory over Fernando Verdasco. "We saw you in 1999 against Andre Agassi.”

“Ah, I won that match,” Martin said.

“Against Agassi?”

“That’s how I go to sleep every night,” Martin said, smiling.

For the record, Martin lost that heartbreaker of a match, a five-set U.S. Open final in which he led two sets to one. That’s as close as he came to winning a major title, something his new pupil has already done. But Djokovic, whose tennis has stalled since he won the 2008 Australian Open, wasn’t looking for a mentor who had won a lot of Slams or once played a glamorous game. The fast, freakishly flexible Serb has plenty of flash. What he needs is someone sturdy, someone measured and thoughtful, someone who thinks—sometimes long and hard—before he speaks.

Martin, nonetheless, was surprised to get the call. He had met Djokovic before, but wasn’t looking to become a coach.

“I just got a call out of the blue,” Martin said. “I sort of felt like professional tennis was in my rearview mirror.”

Martin is not replacing Marian Vjada, Djokovic’s full-time coach, and the particulars of his relationship with Djokovic following the U.S. Open are yet to be determined. But even if Martin travels little, or if the two part ways, Djokovic’s partnership with the former American pro is a milestone in his career. Essentially, it’s an admission that he wants to grow up.

I’m not suggesting that Djokovic needed to leave behind juvenile antics or ditch his impressive array of impressions. Rather, he needed to take better care of his talent, and more interest in honing it. In Melbourne two years ago, men’s tennis seemed primed for a new leader, and Djokovic was the can’t-miss candidate. Over the next two years, though, his game seemed to stand still as Andy Murray packed on a few pounds of muscle, as Juan Martin del Potro toughened up, as Roger Federer recovered from mono and then surpassed Pete Sampras, and as Rafael Nadal improved at, well, everything. Though Djokovic has never fallen below No. 4 in the rankings these past two years, of late he’s been aloof, unpredictable and irritable. Andy Roddick, ranked No. 5, has beaten Djokovic three straight times.

In tennis, as in life, knowing you have a problem is the easy part. Doing something about it takes courage and a little daring, a willingness to experiment and to be proven wrong. By deciding to travel less often with his family and by adding another voice—a new, pragmatic voice—to the practice court, Djokovic is doing what few tennis players do in a time of crisis. He has decided to add rather than subtract, to step back and dispassionately assess his flaws rather than lash out, fire his coach, and become more stubborn and insulated. Martin used to employ a similar approach.

“That’s the way I lived,” Martin said. “My whole career I had Jose Higueras contributing to my development, leading my development at times, and then sort of participating in small doses on occasion. That was essential for me. I think Marian is supportive to the point where he’s happy to have [me], happy to have another set of eyes, a different voice to communicate some things that he’s probably communicated for years, but in a different way.”

Martin wouldn’t delve into specifics when asked about what he would like Djokovic to do that he’s not doing now. “It’s been two weeks,” he said. “I’m learning a lot.”

So far, though, one thing has surprised him about his new pupil: “I underestimated his athleticism, and I had pretty high opinions,” he says. “He’s extraordinarily explosive, but he’s also incredibly coordinated and balanced.” He also said Djokovic could improve at everything.

“He’s a very complete player, but he’s far from complete,” Martin said.

I had one more question for Martin: Why does he take more notes than any coach I've ever seen?

“My recollection is not great,” Martin said. A player who knows his flaws deserves a coach who does, too.

Tom Perrotta is a senior editor for TENNIS magazine. Follow him on Twitter.