TENNIS fans love to argue about the greatest player of all time. When the conversation is limited to the men’s game on clay courts, the debate has long been over. Rafael Nadal, who won his tenth French Open title on Sunday by defeating Switzerland’s Stan Wawrinka, has dominated the surface like no other player in the history of the game. No other man or woman has won a single tournament—major or not—ten times, and only Bjorn Borg holds as many as six titles at Roland Garros. Mr Nadal has now done so at three different clay-court events, recording his tenth title—dubbed “La Décima”—at both Monte Carlo and Barcelona in April.

Like Roger Federer, his longtime rival, who skipped this year’s clay season and returns this week on the grass courts in Stuttgart, Mr Nadal appears to be immune to ageing. During the tournament, the Spaniard celebrated his 31st birthday, an age by which many former greats have retired. Instead, he played as well as ever, losing only 35 games in his seven matches in Paris. That was just short of Mr Borg’s 1978 record of 32, and set a new personal best: his previous low mark was 41, in 2008. (Mr Nadal’s figure was somewhat artificially aided by the fact that his compatriot Pablo Carreño Busta suffered an injury and retired in the middle of their quarter-final match.) It was the third time that Mr Nadal has navigated all the way through the French Open draw without losing a single set.

For Mr Nadal to claim a Roland Garros trophy at age 31 is, in some ways, even more impressive than Mr Federer’s surprise run to this year’s Australian Open title at age 35. Compared with Mr Federer’s brand of first-strike tennis, which served him well at this year’s fast-playing courts in Melbourne, the Spaniard’s game—like those of most clay-court specialists—is extremely physically demanding. Mr Borg played his last major final at age 25, and Gustavo Kuerten, a previous three-time champion in Paris, won his last Roland Garros match at age 27. To reach this point, Mr Nadal needed to bounce back from a recent bout of injury troubles, which forced him to miss the second half of the 2014 season and to withdraw from last year’s French Open after the first two rounds.

Mr Nadal could be forgiven if he elects to borrow from Mr Federer’s playbook and sit out a couple of months while the tour moves to his less-preferred surfaces, hard and grass courts. Over the course of his career, his preference for clay has been significantly stronger than Mr Federer’s liking for anything but clay. According to Elo ratings—an algorithm that evaluates players based on their performances and the quality of their opponents, and can be limited to matches played on a single surface—the gap between Mr Nadal’s clay-court and hard-court performances is about twice as large as Mr Federer’s.

In 2009, just before that year’s French Open, Mr Nadal recorded the highest-ever clay-court Elo score, at 2,543. That exceeded his hard-court Elo mark at the time by 273 points—a differential as big as the chasm between the current world number one by Elo, Novak Djokovic, and Alexander Zverev, ranked tenth. It suggests that the clay-court version of Mr Nadal at his best would beat the hard-court form of the same player 83% of the time.

Mr Nadal’s penchant for clay has remained nearly as strong for most of his career: the gap between his clay- and hard-court Elo scores briefly dipped below 200 points in 2014, but has rebounded to 267 points today. His career-average surface preference is more extreme than that of any other player currently ranked in the top 50. Where you place Mr Nadal in the overall pantheon of tennis greats depends largely on whether you think he should be rewarded or penalised for deriving such a disproportionate share of his value from just one of the sport’s three environments.

By contrast, surfaces have had a much smaller impact on Mr Federer’s game. His Elo scores on hard courts have usually been less than 150 points higher than his marks on clay. And after he won the 2009 French Open—one of only three times since 2005 that the trophy did not go to Mr Nadal— Mr Federer’s clay rating actually surpassed his hard-court number. Of course, that did not make him a better clay-court player than Mr Nadal: the Spaniard’s clay-court Elo mark was still over 200 points higher than Mr Federer’s, implying that Mr Nadal would have a 77% chance of winning a match between them on clay at that point. But it does show that Mr Federer was a more well-rounded player.

It would be unfair to dismiss Mr Nadal as a one-trick clay pony. In the past, single-surface specialists went to great lengths to maximise the share of their matches played on their preferred court type. Guillermo Vilas, the previous record-holder for most titles on clay, played a whopping 60% of his career on that surface. Today, however, scheduling is more rigid: three of the four grand-slam tournaments are played on hard or grass courts, as are six of the nine events in the second tier of the Association of Tennis Professionals (ATP) tour, the Masters 1000 series. And Mr Nadal cannot be accused of selectively skipping events to pad his win rate. Clay-court events represent just a third of his total.

Moreover, Mr Nadal has proven that he is still an all-time great during the majority of his matches that occur on hard or grass courts. He has won five majors on other surfaces. Taking the average ratings from the eight-year span when their rivalry was at its peak, from 2006 to 2013, Mr Federer led Mr Nadal in hard-court Elo by a respectable 141 points (2,320 to 2,179), while Mr Nadal led in the clay-court category by a much larger 227 points (2,496 to 2,269). Those ratings imply odds of 69% for the Swiss on hard courts, but 79% for the Spaniard on clay. Mr Nadal outperformed those probabilities on both surfaces—he won seven of twelve matches on hard courts and twelve of fourteen on clay—but the ratio is about right.

Now that Mr Nadal is over 30, it is tempting to assume that his days of contending for titles in non-clay events are over. However, his hard-court Elo score still ranks fourth in the world, behind Mr Djokovic, Mr Federer and Andy Murray, the defending Wimbledon champion. And although grass-specific ratings are less reliable, because relatively few events are played on turf, Mr Nadal did go on to win Wimbledon in each of the past two years that he claimed victory at Roland Garros without dropping a set. The clay-court master may not have to wait until next spring to improve his case to be considered the greatest of all time—on any surface.