Rafael Nadal won Indian Wells. Roger Federer won on the blue clay. Even Alessio Di Mauro won a match on a hard court last week.
That’s just a sliver of the anecdotal evidence for one of the most common complaints about contemporary ATP tennis: Surface speeds are converging. Hard courts used to play faster, allowing for more variety in the game and providing more opportunities to different types of players. Or so the story goes.
This debate skipped the stage of determining whether the convergence is actually happening. The media has moved straight to the more controversial subject of whether it should. (Coincidentally, it’s easier to churn out columns about the latter.)
We can test these things, and we’re going to in a minute. First, it’s important to clarify what exactly we mean by surface speed, and what we can and cannot learn about it from traditional match statistics.
There are many factors that contribute to how fast a tennis ball moves through the air (altitude, humidity, ball type) and many that affect the nature of the bounce (all of the same, plus surface). If you’re actually on court, hitting balls, you’ll notice a lot of details: how high the ball is bouncing, how fast it seems to come off of your opponent’s racket, how the surface and the atmosphere are affecting spin, and more. Hawkeye allows us to quantify some of those things, but the available data is very limited.
While things like ball bounce and shot speed can be quantified, they haven’t been tracked for long enough to help us here. We’re stuck with the same old stats — aces, serve percentages, break points, and so on.
Thus, when we talk about “surface speed” or “court speed,” we’re not just talking about the immediate physical characteristics of the concrete, lawn, or dirt. Instead, we’re referring to how the surface–together with the weather, the altitude, the balls, and a handful of other minor factors–affects play. I can’t tell you whether balls bounced faster on hard courts in 2012 than in 1992. But I can tell you that players hit about 25% more aces.
Quantifying the convergence
In what follows, we’ll use two stats: ace rate and break rate. When courts play faster, there are more aces and fewer breaks of serve. The slower the court, the more the advantage swings to the returner, limiting free points on serve and increasing the frequency of service breaks.
To compare hard courts to clay courts, I looked for instances where the same pair of players faced off during the same year on both surfaces. There are plenty–about 100 such pairs for each of the last dozen years, and about 80 per year before that, back to 1991. Focusing on these head-to-heads prevents us from giving too much weight to players who play almost exclusively on one surface. Andy Roddick helped increase the ace rate and decrease the break rate on hard courts for years, but he barely influences the clay court numbers, since he skipped so many of those tournaments.
Thus, we’re comparing apples to apples, like the matches this year between David Ferrer and Fabio Fognini. On clay, Ferrer aced Fognini only once per hundred service points; on hard, he did so six times as often. Any one matchup could be misleading, but combine 100 of them and you have something worth looking at. (This methodology, unfortunately, precludes measuring grass-court speed. There simply aren’t enough matches on grass to give us a reliable sample.)
Aggregate all the clay court matches and all the hard court matches, and you have overall numbers that can be compared. For instance, in 2012, service breaks accounted for 22.0% of these games on clay, against 20.5% of games on hard. Divide one by the other, and we can see that the clay-court break rate is 7.4% higher than its hard-court counterpart.
That’s one of the smallest differences of the last 20 years, but it’s far from the whole story. Run the same algorithm for every season back to 1991 (the extent of available stats), and you have everything from a 2.8% difference in 2002 to a 32.8% difference in 2003. Smooth the outliers by calculating five-year moving averages, and you get finally get something a bit more meaningful:
The larger the difference, the bigger the difference between hard and clay courts. The most extreme five-year period in this span was 2003-07, when there were 25.4% more breaks on clay courts than on hard courts. There has been a steady decline since then (to 16.9% for 2008-12), but not to as low a point as the early 90s (14.0% for 1991-1996), and only a bit lower than the turn of the century (17.8% for 1998-2002). These numbers hardly identify the good old days when men were men and hard courts were hard.
When we turn to ace rate, the trend provides even less support for the surface-convergence theory. Here are the same 5-year averages, representing the difference between hard-court ace rate and clay-court ace rate:
Here again, the most diverse results occurred during the 5-year span from 2003 to 2007, when hard-court aces were 51.3% higher than clay-court aces. Since then, the difference has fallen to 46%, still a relatively large gap, one that only occurred in two single years before 2003.
If surfaces are converging, why is there a bigger difference in aces now than there was 10, 15, or 20 years ago? Why don’t we see hard-court break rates getting any closer to clay-court break rates?
However fast or high balls are bouncing off of today’s tennis surfaces, courts just aren’t playing any less diversely than they used to. In the last 20 years, the game has changed in any number of ways, some of which can make hard-court matches look like clay-court contests and vice versa. But with the profiles of clay and hard courts relatively unchanged over the last 20 years, it’s time for pundits to find something else to complain about.
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