John Isner became the top-ranked American male tennis player by playing his best tennis at home. He wins more than two-thirds of his matches in the U.S., but just half elsewhere. Tennis writers have portrayed Isner’s strength at home as a weakness abroad. But in his sport, where players set large parts of their own schedules, displaying a repeatable competitive advantage is an opportunity, not a liability.1Unlike, say, in the NBA, where an Eastern Conference team that struggles out west can’t replace trips to California with more home dates.Even as he’s pledged to solve his road woes, Isner has filled his calendar with U.S. events. His home-court advantage has helped him rise this month from the world’s No. 13 to No. 10. A couple of weeks ago at a tournament in Indian Wells, Calif., Isner reached the semifinals, where he took a set off No. 2 Novak Djokovic. This week in Miami, he reached the round of 16 but lost on Tuesday to No. 7 Tomas Berdych. In two weeks, Isner will seek to defend his title in Houston.These wouldn’t have passed for spectacular American results when Andre Agassi and Pete Sampras ruled the sport in the 1990s, or even when Andy Roddick and James Blake took up residence in the top 10 during the last decade. These days, though, pretty good is as good as it gets for American men in tennis. None of Isner’s peers got past the round of 64 at either tournament this month; he was the last American man at each by at least two rounds. And no other American man is ranked in the top 60 in the world. (There’s little reason to hope for better things from the next generation: No American ranks in the top 20 in either the under-20 or under-21 world rankings.)Isner is famous among casual fans for his role in the longest match ever played, which he won over Nicolas Mahut at Wimbledon in 2010, with the basketball-like 70-68 score in the fifth set. But he’s done his best work at home. Fourteen of his 17 career finals and six of his eight career titles have come in the United States. He’s been an entirely average player at the tour level2This means matches that count towards a player’s official match record: matches at Grand Slam tournaments, in Davis Cup matches and at ATP World Tour events. away from the U.S., winning 51 percent of his matches. At American events, he’s won 69 percent.“I always play my best in the United States,” Isner said at a press conference in Indian Wells. “A lot of times, especially in Europe, I have ‑‑ you know, I haven’t had great results at all.” He was at a loss to explain why, offering perhaps a lack of toughness at overseas tournaments. “There is no reason I can’t have a result like this outside of the U.S.,” he said.The reasons for Isner’s home advantage are varied. The obvious suspects, like the surface he’s playing on and the strength of his opponents, don’t fully explain it. A lot of it comes down to Isner himself.It’s true that much of Isner’s home success has come against weak competition. He has thrived at smaller U.S. tournaments that are optional for top players, who mostly live in Europe and don’t bother to make the trip. These events account for all of his U.S. titles and all but two of his U.S. finals. Just 6 percent of his matches at those events have come against top 10 players, none ranked in the top four. The relative weakness of his competition thanks to these events can be seen in the median ranking of his opponents over the last year: just 64, making his the softest schedule of any player in the top 35 in the world rankings.Isner also gets to play on hard courts, his favorite surface, at most of the U.S. events where he chooses to play. Just two are played on other surfaces: Houston, on clay; and Newport, R.I., on grass.These factors alone don’t explain Isner’s U.S. success, though. I pulled his career match record and ran a logistic regression, controlling for surface,3Isner has played 32 matches on grass, 66 matches on clay and 256 matches on hard courts. I separately ran the regression with each surface and also combining hard and grass, since so few matches are played on grass. The results were essentially the same. the ranking of his opponent4Technically I used the logarithm of his opponent’s ranking, since there is a much wider gap between the No. 1 and No. 10 players in the world — and therefore the probability of beating each one — than there is between the No. 10 and No. 100 players. and the value of each match, in ranking points.5The goal was to check whether Isner plays better in higher-leverage matches, those that count for more — i.e. matches in big tournaments, or later rounds of smaller ones. If he does, this effect could be confused with a preference for home courts. That’s because many of his U.S. events have weak fields, pitting Isner against early-round opponents whom he’d likely beat anywhere. That gives him more high-stakes home matches, so if he thrives in high-stakes matches, it might help explain his home advantage.To calculate the leverage of each match, I took the number of ranking points Isner would receive if he lost the match and subtracted it from the number he would get if he won, then lost the subsequent match. The result is roughly the value of the match, as prize money rises with ranking points and the points also determine a player’s subsequent seedings and affect his earning potential. The calculation is complicated by the ATP’s change in ranking points in 2009, so it isn’t exact, but since most of Isner’s tour-level matches came after 2008, the effect is small. Even after controlling for these factors, Isner remains a homecoming king. Surface, it turns out, isn’t a statistically significant driver of his success. Nor is the value of winning the match. His opponent’s ranking is highly significant. But independent of these factors, a 50-50 match for Isner away from home becomes a match he’ll win two out of three times in the U.S.Tennis isn’t usually associated with strong home-court effects, because of its individual and international nature. Many events draw fans from across the globe, who cheer for players from countries other than their own. And most players get few chances to play at home outside of the Davis Cup, the partisan international team competition that provides a rare home-court advantage in tennis. A popular explanation for home advantage in many other sports — that officials are influenced by partisan crowds — doesn’t translate to tennis because electronic line-call review at the sport’s top levels has greatly reduced the potential influence of subjective calls on match outcomes.Perhaps Isner thrives so much at home because of his background in college tennis, a level of competition where the team is primary. Isner starred at the University of Georgia and loves college team sports, spending much of a press conference last Saturday in Miami breaking down his NCAA men’s basketball tournament bracket. Isner counts on support from American crowds, and was taken aback by U.S. Open fans’ cheers for his opponent, Frenchman Gael Monfils, last summer.Isner lamented his inconsistency away from home in that Indian Wells press conference, and he’d naturally rather do as well outside the U.S. as he does in it. But if he had to choose between his unbalanced current record and, say, maintaining the same win probability everywhere, he should opt for the status quo. Ranking points and prize money nearly double at each stage of a tournament, rewarding players who alternate finals with first-round exits over players who consistently lose in the second round.6We can illustrate this by imagining a simplified five-tournament sequence in which each tournament has 32 players and five rounds. Points and prize money double each round, from one point and $1 for a first-round exit up to 32 points and $32 for a title.Player A, with one title and four first-round losses, would pick up 32 points and $32 for the title, and an additional four points and $4 for the other four tournaments, for a total haul of 36 points and $36. His record would be 5-4.Player B, with five quarterfinal exits, would get four points and $4 in each tournament, for a total of 20 points and $20 — barely half the yield of Player A, despite a superior win-loss record of 10-5.So inconsistency in tennis is good. Even better is predictable inconsistency. A player who doesn’t know when he’ll thrive can’t plan around it. Someone who does best at clay-court events can schedule as many as he can fit in. A player who plays best at home ought to schedule as many home tournaments as possible. Isner has learned that lesson. He has reaped the benefits of a tournament calendar that still features a significant number of U.S. events, even as players from other countries have ascended in the rankings.In addition to the U.S. Open and the mandatory events in Indian Wells, Miami and Cincinnati, Isner had 10 ATP events in the U.S. to choose from in 2007 and 2008, his first two years on tour. That number declined to nine, then eight and then, this year, seven. But the decline in American men’s talent has been even steeper during that time, making ranking points at those events low-hanging fruit for Isner. Combine the easy fields with his home-court preference, and Isner finds lots of success in places such as Atlanta, Winston Salem, N.C., and Houston — even as events he played earlier in his career in Indianapolis, Las Vegas, San Jose, Calif., and New Haven, Conn., have vanished.Early in his career, Isner didn’t choose so well for himself. In his first two years on tour, he opted to play just three of his 10 non-mandatory events in the U.S. But from 2009 to 2013, he managed to play 29 of his 53 optional events in the U.S., even though only one-fifth of such events took place there. Last year, the U.S. hosted eight of these events, and Isner played in seven. He reached the semifinals of six and the finals of three, winning twice.Isner has taken advantage of his home-court preference more wisely than his peer and frequent doubles partner, Sam Querrey. I ran the same analysis on Querrey, the second-ranked American man today. For Querrey, too, surface and leverage weren’t significant. He also showed a significant home-court advantage, though the effect was smaller and less significant than for Isner.7A 50-50 match away from the U.S. for Querrey would turn into a match he’d win 62 percent of the time at home. Yet after playing almost exclusively at home in his rookie year on tour, Querrey has opted to play events away from the U.S. almost as often as home tournaments, averaging one more optional road trip per year than Isner.Perhaps many players would show a strong, significant home advantage if they had the chance. None of the world’s top five players gets more than two or three home events each year. Players from the other Grand Slam-hosting countries — the U.K., France and Australia — have a few more opportunities. But those countries combined have about the same number of tournaments as the U.S.Tennis’s general move away from the U.S., and Isner’s impending 29th birthday, might keep him from entering as many home events in the future. He’s compensating by making more of his opportunities and stepping up at the bigger U.S. events, such as this month’s strong runs and his finals in Cincinnati last year and in Indian Wells the year before that. If Isner can keep improving at the big U.S. events, he won’t have to worry about getting better away from home.