Stroke Differential: A New Statistic Reveals the Key to Rafael Nadal’s Success
The use of statistics to understand results in professional tennis is in its infancy. A positive development, led by the pioneering work of Craig O’Shannessy, investigates rally length and its influence on outcomes. Craig’s finding that the top players are more successful in the 0-4 shot rallies (when compared with 5-8 and 9+ shot rallies) has, no doubt, changed the way that top players practice and strategize.Another way of finding out more about what really happened in a match is to look at point-ending strokes. When doing so it’s important to remember that point-ending strokes include not only winners/forced errors, but they also include unforced errors. The difference between winners/forced errors and unforced errors is called the STROKE DIFFERENCE. A positive DIFFERENCE would mean a particularly effective stroke, while a substantial negative DIFFERENCE would mean that a stroke has been broken down.
A particular stroke’s DIFFERENTIAL is the difference of the DIFFERENCES of the two players for a particular stroke. This allows us to see the relative strengths of the strokes being compared. The DIFFERENTIAL is a simple metric which asks the following question: if we compare the effectiveness of any particular stroke, which player comes out ahead? For example, in their Wimbledon semi-final Federer had a FH DIFFERENCE of +3, while Nadal had a FH difference of -2. This gives Fed a FH DIFFERENTIAL of +5 (thus a -5 for Rafa). Looking at the STROKE DIFFERENTIAL can provide insights into a player’s relative strengths and weaknesses.
Here are Nadal’s BH stats from recent matches against high-quality opponents:
Opponent Surface Result DOMINANCE DIFFERENTIAL
Federer Grass Loss 14.81% +9
Djokovic Clay Win 12.35% +18
Federer Clay Win 16.02% +11
Thiem Clay Win 16.67% +12
Tsitsipas Clay Win 12.5% +16
Nadal’s BH DIFFERENTIAL, provides a big clue as to how he wins. In these 5 matches against high-quality opponents, Rafa was consistently able to break down their backhands. A +13.2 is a nearly impossible margin for an opponent to overcome. By contrast, Nadal’s FH DIFFERENTIAL average for these 5 matches is +7.6. This is still strong, but it pales in comparison with his BH number.
Serving is another matter. Rafa’s serving DIFFERENTIAL number is -.2. This means that he is essentially staying even with the serves of these 5 strong opponents. Since both of his groundstrokes have a significant positive DIFFERENTIAL, a breakeven in the serving category is no big deal for Rafa.
How well do these stats hold up in illuminating the US Open final against Medvedev? Here are the numbers:
Stroke PLAYER DOMINANCE DIFFERENTIAL
FH Nadal 19.65% +6
BH Nadal 11.44% +12
S Nadal 7.62% -5
Nadal used the same formula to win the Open! His FH and BH DIFFERENTIALS are right in line with his averages. And, while his serve lagged that of Medvedev slightly, it also did so to the same degree against both Djokovic and Tsitsipas (-5 in both matches). He won both of these matches.
Using information from point-ending strokes also suggests looking at another statistic: STROKE DOMINANCE. DOMINANCE is simply the percentage of point-ending shots attributable to the basic strokes in tennis: forehands, backhands, serves, and volleys & overheads. For example, Roger Federer’s FH DOMINANCE number is typically between 22 AND 25%. This means that very close to a quarter of all points played in a match end with Fed hitting a FH.
Do these statistics offer us any way of seeing why Nadal lost to Federer at Wimbledon? They sure do! Nadal’s FH DOMINANCE number against Fed was 13.99%. In each of his victories, it was a level of magnitude higher.
Opponent DOMINANCE DIFFERENTIAL
Djokovic 22.22% +10
Federer 19.89% +14
Thiem 19.19% +10
Tsitsipas 23.21% +9
Medvedev 19.65% +6
In the Wimbledon semi, Nadal had a -5 FH DIFFERENTIAL. It could have been the surface or just a bad day at the office – it’s hard to tell. What is clear it that he played too passively and paid the price. And what is even clearer is that it’s Rafa’s BH that is his decisive advantage. It outperforms the BH’s of his opponents each and every time!