The red dirt of Roland Garros, which has held the attention of the tennis world this week, is the most defining feature of the only clay-court Grand Slam. But, when it comes to performance, is clay losing its distinctiveness? In this guest post, Graeme Spence, delves into the closing gap in surface specialisation.
Historically, the back-to-back Majors of Roland Garros and Wimbledon offered tennis fans the sharpest contrast of styles. Months after the red dirt in Paris showed the mastery of the best spin-heavy ralliers in the game, the green courts at Wimbledon would showcase the power of the hardest hitting servers and the finesse of the best net games in the sport.
As much as the contrast in colour palette, the contrast in styles have been an essential part of the tennis calendar.
A small twitter storm kicked up this week when an article from ESPN criticised this tradition, arguing that the number of clay court tournaments on the calendar is too many. Whether a reasonable argument or not, the main point of that piece presumes that play on clay courts is fundamentally different than on hard courts, the surface played at the majority of events.
A close viewer of tennis over the past decade can’t help but have the impression that, as the baseline game has taken hold of the sport, the prevalence of surface specialists has appeared to dwindle. But do the numbers back up this impression?
Surface-Specific Elo Ratings
There are many ways one could define ‘surface specialisation’. To understand broader trends, my team at Tennis Australia’s Game Insight Group decided to focus on the differences in ability of the top 100 ranked players on each surface from 1990 to 2017. Specifically, an individual player’s surface-specific Elo rating was compared against the average rating of the top 100 players that year.
This approach allowed us to judge the ability of a player on different surfaces relative to their peers, by comparing—for example—their relative Clay Elo rating against their relative Hard Elo rating. For fairness, we excluded ratings from the dataset if a player hadn’t played at least one match on that surface in that year. For the men, this excluded on average 2 players' hard court Elo ratings each year, 4 players' clay Elo and 16 players' grass Elo. For the women, the average rates of exclusions were less than one player for the hard Elo ratings, 3 for clay and 10 for grass.
ATP trends in surface specialisation
When we look at the mean absolute differences between the relative clay and hard Elo ratings for each year, there is a clear reduction in the difference between men’s clay and hard abilities. The trend is consistent over the last 28 years: with the average rating difference starting at around 175 points and decreasing to its current level of about 110 points. This trend is observed both for players with higher clay ratings than hard, and those that are better on hard than clay. To put these values into context, an Elo points difference of 175 corresponds to a 73% single-match win probability for the higher rated player, and a 110 points difference corresponds to 65%.
There also appears to be a reduction in the average difference between grass and hard relative abilities, although the trend is weaker when compared with the trend for clay. The average difference between grass and hard abilities was at 130 points in 1990, considerably lower than the clay difference. And a more gentle reduction is observed to the present day difference of about 100 points.
WTA trends in surface specialisation
On the women’s side, similar reducing trends are observed for both clay and grass. For both surfaces, the average difference in relative ability have reduced from 130 Elo points to about 100-110 points. Again for context, an Elo points difference of 130 corresponds to a 68% single-match win probability for the higher rated player, and a 100 points difference corresponds to 64%.
Effect of scheduling changes
Complications arise when considering surface-specific abilities due to differences in players' schedules between surfaces and scheduling changes over time. Firstly, as more players miss the grass court season than the clay, we would expect more uncertainty in the grass court trends. Secondly, while the number of top 100 players missing the clay court season stays relatively stable for both tours over this time period, the number of players skipping grass court play reduced considerably: from a high of 29 in 1990 to a low of 7 in 2011 for the men, and from 19 in 1991 to 2 in 2015 for the women.
How could this affect the above trends? If players in the past who were worse on grass opted out of the grass-court circuit1, the true average difference between grass and hard court abilities would be greater than we are estimating. In which case, we would be underestimating the strength of the decline in grass court specialisation for both men and women. Unfortunately it is difficult to draw any firm conclusions on either the potential underestimation in the early part of our trends or the gender differences, due to the uncertainty around the extent of any selection bias.
Nadal: supreme clay court specialist but far from a one-trick pony
Although the average level of surface specialisation appears to have diminished across the board—most strongly for the men on clay—this isn’t to say that individual specialists don’t exist. Rafael Nadal’s unparalleled exploits on clay definitely qualify him as a supreme clay court specialist, which is reflected in his remarkably high clay Elo rating over the past two decades. But it’s well-documented that he has worked hard to improve his hard court game over the years. After 2005, when he he burst onto the scene by winning his first French Open as a teenager, his difference in relative clay and hard court Elo ratings shows a clear reduction, agreeing with the global trend of reduced surface specialisation.
With 10 French Open titles to his name (and counting) it would be hard to argue a reduction in Nadal’s clay ability has occurred, more likely a substantial improvement in his hard court level—as his four hard-court Grand Slams between 2009 and 2017 testify.
Roger Federer’s choice to skip the 2017 and 2018 clay court seasons is an interesting modern example of a player opting out of his ‘worst’ surface, even though what is ‘worst’ for Federer would be an enviable level to most of the tour. ↩︎