Isner, 35, is the lone American in the top 30. In the 1990s, just as tennis was becoming a truly global sport, Pete Sampras, Andre Agassi and Jim Courier were mainstays of the top 10. Canada, which is about one-tenth of the size of the United States, has three men in the top 20. Also, the Australian Open takes place on a hardcourt, the surface that most Americans grow up playing on.“I think I’m more than capable, but it’s a matter of not what I do against Novak but what do you do every day,” Tiafoe, 23, said after he had lost his hard-fought four-set, three-and-a-half-hour battle with Djokovic. “Those matches, losing matches, I don’t think I should.”Fritz came a step closer to beating Djokovic on Friday night, pushing him to five sets as Djokovic struggled through an injury he described as a torn muscle on the right side of his midsection. Fritz appeared to have Djokovic beaten early in the fifth set but fell short as Djokovic began pounding serves and ripping forehands into the corners, as he had early on in the match.An hour after it ended, Fritz remained distraught over too many missed first serves and errors off his forehand. He had taken Djokovic to a tiebreaker in the first set and had then lost seven of the next eight points.“It’s very motivating that we’re so close, but at the same time, we are so far,” Fritz said. “These guys are so good.”And so it was that McDonald, perhaps the most unlikely of all of his countrymen, became the last hope to put an American into the second week of the year’s first Grand Slam. McDonald showed promise three years ago when, not long after leaving U.C.L.A., he made it to the fourth round of Wimbledon, where he lost to Milos Raonic of Canada.Less than a year later, he sustained a torn hamstring tendon while playing doubles at the French Open and underwent surgery. After the operation, he couldn’t leave his apartment for three weeks, and he couldn’t walk for the better part of two months. Slowly, week by week, he began to allow his leg to bear more weight.