“I love the kid’s attitude,” McEnroe­ says of Rubin. “He has really good tennis I.Q. He’s extremely solid. The one thing missing, of course, is size. I mean, you look at the picture of him standing with Kozlov at Wimbledon, and Stefan’s only 16 but he’s a good two or three inches taller. And people say Stefan is short.”

Kleger, however, sees a precedent for someone of Rubin’s stature and hopes he will develop into “some combination of David Ferrer and Marcelo Rios.” Ferrer is known for a tenacious and disciplined style that has kept him in the Top 10 well into his 30s. Rios was an offensive genius who reached No. 1 in the world in 1998, largely by taking the ball on the rise and, Kleger says, “making players who hit with more pace uncomfortable, because he robbed you of time.” Both are 5-foot-9. “Now, whether there’s room for someone like that in 10 years, who knows?”

A few weeks after Wimbledon, Rubin was on the distinctly unglamorous Futures circuit, where he and most of his peers will most likely spend the first several years of adulthood. The competition was in Godfrey, Ill. — “the kind of tournament almost designed to make you ask yourself if you really want to be doing this,” he said. It was held on the campus of a local community college, where the tennis pavilion had no locker room, just a single men’s room for players and officials to share. The total payout — for those competitors who weren’t playing N.C.A.A. tennis during the school year or didn’t plan to — was just $10,000, with prizes ranging from $117.50 for each first-round loser in men’s singles to the champion’s purse of $1,300.

To save on expenses, Rubin’s first-round opponent, Spencer Papa, was staying in the spare bedroom of local tennis boosters, whose teenage children were on hand to ferry him and another player to Walmart to stock up on Pedialyte. Rubin’s airfare and motel expenses were covered by the John McEnroe Tennis Academy on Randalls Island, which sent a traveling coach, Mark Bowtell, to keep him company.

Rubin’s success over the summer had put a target on his back. “Everyone wants to take a stab at him,” Bowtell said, adding that Rubin told him, “Now when someone beats me, they’ve beaten a Wimbledon champion.” Rubin got off to an easy start but fell apart. He won the first set 6-2, led the second 4-0 but lost it 7-5, and then dropped the final set 6-1. Papa said Rubin seemed to ease up just as he was about to walk away with the match. “I was mentally kind of down,” Papa told me, “but I had a lucky game when I held serve at 3-4, even though he was hitting good returns.” After that, Rubin tensed up. “He stopped coming forward on the baseline the way he normally does. I couldn’t tell what happened to him.” Papa added: “Even from the outset, he seemed like he was carrying himself differently, probably because of Wimbledon. We’re pretty good friends, and he’s always been a confident guy, but — well, I’d just say that everybody in the tournament came up to me after and said, ‘I’m so glad you beat him.’ ” (Eric Rubin says the reason his son lost was that he came down with a virus.)