The Slow but Steady Erosion of the Server’s Advantage

After a couple of weeks of data-driven skepticism, I can finally confirm a bit of tennis’s conventional wisdom. Over the course of a typical match, breaks of serve are a little easier to come by.

This result–based on tens of thousands of matches from the last few years–is similar for both men and women. After about twelve games (total, not service games for each player), a hold is roughly 2% less likely than it was in the first few games of the match. By the 25th game, a hold is approximately 5% less likely than at the beginning of the match.

To control for the vagaries of surface, opponent, and other conditions, I’ve compared each service game to the server’s hold percentage within that match. Only the closest matches are likely to go very long, so it’s important to compare the last games of those matches to games with similarly even opponents.

It seems that this effect is the result of one or both of two factors: server fatigue (which may have more of an effect on results than an equivalent amount of returner fatigue), and the returner’s increasing familiarity with the server. It would be difficult to separate these two–and with this dataset, probably impossible–so for today, let’s stick with the nature of the effect, not its causes.

The following graph shows the relative probability of a hold of serve based on how much of the match (in games) has been played:

Relative hold percentage

I’ve set the hold probability of the first game at 100%, so all other numbers are relative to that. I’ve excluded tiebreaks from these calculations, though I considered them when counting games–that is, the first game of the second set after a tiebreak is considered the 14th game, not the 13th.

The results get a lot noisier starting around the women’s 25th game and the men’s 35th game, for the simple reason that most matches don’t get that far. For example, while the WTA calculations are based on 11,000 matches, only one-third reached the 25th game and less than one-tenth made it to the 31st.

The general downward trend indicates that the fatigue and/or familiarity effect dwarfs the effect of new balls. I have found that in men’s matches, the age of balls has a very small effect on hold percentage, and in women’s matches, it has no effect. In any case, the steady ebb of the server’s advantage is a stronger effect.

It is likely that some players suffer more from fatigue or familiarity than others. Due to the smaller size of the per-player samples, especially beyond the 20th game or so, I’m reluctant to draw any strong conclusions. Still, there are some intriguing numbers for the players for whom the dataset contains the most matches.

Here, I’ve calculated the hold percentage for several top players at various stages of the match, relative to their hold percentage in the first ten games. Thus, a number below 100% indicates less frequent holds, while a number above 100% means more frequent holds:

Player                 Matches  11 to 20  21 to 30  31 to 50  
Tomas Berdych              337     98.5%     98.3%    101.5%  
David Ferrer               330     97.0%     99.4%    102.4%  
Novak Djokovic             325    100.1%    101.8%    101.7%  
Roger Federer              325    100.2%     99.6%    100.4%  
Andy Murray                295     97.7%     98.7%     97.9%  
Rafael Nadal               293     99.2%    100.3%     93.7%  
Jo-Wilfried Tsonga         255    100.4%    100.9%     99.6%  
Philipp Kohlschreiber      252    101.4%     97.9%     96.7%  
John Isner                 251    100.4%    100.4%    100.3%  
                                                              
Player                 Matches  11 to 20  21 to 30  31 to 50  
Kevin Anderson             247    100.0%     98.1%     97.5%  
Richard Gasquet            246     99.1%     98.4%    105.1%  
Gilles Simon               245    100.1%    103.7%     95.0%  
Milos Raonic               238     97.1%     96.1%     96.7%  
Marin Cilic                238     95.4%     97.5%     94.5%  
Fabio Fognini              235    100.4%     99.6%     98.2%  
Kei Nishikori              233    101.8%    104.1%    107.2%  
Grigor Dimitrov            224    100.9%    100.3%     94.6%  
Andreas Seppi              221    106.4%    100.4%    103.1%  
Feliciano Lopez            221     99.2%     99.7%     98.4%  
                                                              
Total                    23326     98.1%     96.1%     95.1%

While John Isner is steady throughout the stages of the match, other big servers such as Milos Raonic and Marin Cilic are less dominant as the match progresses. The players whose hold percentage improves through the match–such as Novak Djokovic and David Ferrer–tend to be those without big serves, so we may be looking at more of an overall fatigue effect in those cases.

The most extreme number in the table is Rafael Nadal‘s relative hold percentage after the 30th game. Perhaps after that much time on court, his opponents finally figure out how to defend against the ad-court slider.

Here are the same calculations for top WTA players:

Player                Matches  11 to 15  16 to 20  21 to 40  
Agnieszka Radwanska       299    101.0%    104.9%     98.0%  
Sara Errani               279     97.7%     91.2%     92.7%  
Caroline Wozniacki        279    103.1%    102.3%    104.9%  
Serena Williams           266    102.8%    102.4%    104.9%  
Angelique Kerber          265    101.9%    103.0%    101.5%  
Samantha Stosur           253     99.2%    105.0%     97.6%  
Carla Suarez Navarro      252    102.2%    101.8%     93.7%  
Petra Kvitova             251     93.9%    100.4%     95.9%  
Roberta Vinci             250     94.2%     97.9%     95.4%  
Ana Ivanovic              241    100.8%    106.0%     95.2%  
Jelena Jankovic           241    102.2%    108.7%     96.4%  
                                                             
Player                Matches  11 to 15  16 to 20  21 to 40  
Maria Sharapova           236    100.1%    105.9%    104.9%  
Victoria Azarenka         228    100.6%    103.7%     97.8%  
Lucie Safarova            227    102.7%    100.5%     94.4%  
Simona Halep              224     89.2%     95.3%    101.7%  
Dominika Cibulkova        210     98.7%     89.9%     99.9%  
Alize Cornet              210     96.2%    102.8%     96.4%  
Andrea Petkovic           194    101.5%    104.2%    107.5%  
Sloane Stephens           185     97.5%     90.1%     88.7%  
Sabine Lisicki            185     97.4%     97.5%     96.6%  
Ekaterina Makarova        185     96.6%    102.8%     92.8%  
Flavia Pennetta           180    105.1%     92.9%    103.9%  
                                                             
Total                   22406     98.6%     97.2%     95.0%

Here is some confirmation that Serena Williams–at least on serve–gets better as the match progresses. Many of the other players with the strongest serve results late in matches are those known for fitness (like Caroline Wozniacki) or steeliness (Maria Sharapova).

Whether the root cause is fatigue or familiarity, most players are less effective on serve as the match progresses. With further research, I hope we’ll be able to better understand the cause and determine whether there are advantages to serving particularly well at certain stages of the match.