Italian translation at settesei.it
Caroline Wozniacki is set in her habits. In the eight service games of her first round match in Charleston against Laura Siegemund last week, she followed a strict pattern: wide serve on the first point, T serve on the second, T on the third, and wide on the fourth. Aside from two missed first serves that weren’t classified as “wide” or “T”, that’s 30 points. Wozniacki served in her preferred direction on all 30. From the fifth point in each game, her choices were closer to random.
This is nothing new for the Danish former No. 1. Against Monica Niculescu in the Miami third round, she had 11 service games. In the first four points of each, she followed the exact pattern: wide/T/T/wide. 44 service points, and zero deviations from the first-serve script. The Match Charting Project (MCP) has logged over 2,600 WTA matches, and no other player has ever gone an entire match without varying their first-four-point serve direction. Wozniacki has done so 17 times.
Measuring serve predictability
Just how extreme is Caro’s reliability, and how much does she differ from the competition? Let’s take a look.
I classified each first serve as either “wide” or “T.” MCP coding provides for three categories (wide, body, and T), and where a serve is coded as “body,” I used the returner’s first shot as an indication of the serve direction. That’s not perfect, because some returners will run around a weak serve, but it gets us pretty close. I excluded unreturned body serves and body serve faults. Here is Caro’s percentage of wide serves for each point of over 1,000 charted service games:
Point Wide%
1st 82.8%
2nd 17.4%
3rd 16.7%
4th 78.5%
5th 52.3%
6th 46.8%
deuce 48.0%
ad 50.6%
Wozniacki only varies her first serve direction on the first four points about once every five deliveries. If we convert the first four rates (82.8%, 17.4%, 16.7%, and 78.5%) to the frequency with which she hit her favored serve (82.8%, 82.6%, 83.3%, 78.5%), we get an average–call it FSP, for First Serve Predictability–of 81.8%. Only two other women with at least ten charted matches, Kateryna Kozlova and Justine Henin, exceed 70%, and Henin’s repetition has more to do with her preference for the T serve in all situations.
Amazingly, Caro’s overall numbers obscure just how often she uses the pattern these days. The MCP has 52 Wozniacki matches dating from the beginning of 2017, and that more recent subset gives us a FSP of 94.0%. I suspect that the more extreme number is a better representation of Woz’s tendencies, because the more recent data includes a broader selection of matches, including contests against weaker opponents. The MCP is not a random sample, and older matches tend to be more notable ones involving higher-quality opponents.
Wozniacki’s not-really-peers
Let’s take a look at some of the other women who are more predictable than average. The median WTAer with at least 10 charted matches in the MCP dataset has an FSP of about 58%, meaning that they might prefer one direction to the other, or that they often aim for a right-hander’s backhand, but that they vary the first serve delivery quite a bit.
Here are the 20 who change direction the least. For each player, the following table shows the frequency with which they hit a wide serve on each of the first four points, their FSP on the first four points–FSP(1-4)–and their FSP on points from the fifth onward, FSP(5+).
Player 1st 2nd 3rd 4th FSP(1-4) FSP(5+)
Wozniacki 83% 17% 17% 79% 82% 52%
Kozlova 60% 35% 10% 73% 72% 64%
Henin 38% 11% 57% 25% 71% 66%
Vikhlyantseva 92% 46% 38% 63% 68% 54%
Petkovic 74% 72% 36% 38% 68% 58%
Vondrousova 15% 63% 30% 54% 68% 68%
Brengle 82% 67% 53% 68% 67% 56%
Clijsters 86% 32% 61% 52% 67% 56%
Stephens 76% 21% 53% 46% 65% 62%
Voegele 71% 35% 59% 34% 65% 60%
Player 1st 2nd 3rd 4th FSP(1-4) FSP(5+)
Dementieva 76% 54% 71% 60% 65% 60%
Dodin 58% 14% 43% 43% 65% 64%
Li Na 28% 33% 52% 33% 65% 56%
Kerber 43% 78% 56% 67% 65% 64%
Doi 21% 60% 64% 56% 65% 63%
Vandeweghe 35% 35% 62% 66% 65% 55%
A Beck 59% 24% 45% 33% 64% 61%
Sanchez V 43% 77% 42% 65% 64% 64%
Buzarnescu 19% 39% 58% 46% 64% 59%
Sevastova 73% 58% 37% 60% 64% 55%
Only two servers, Kozlova and Natalia Vikhlyantseva, follow the general principle of Wozniacki’s wide/T/T/wide pattern. Many of these players, like Henin, prefer wide or T serves at all times, and others, including Andrea Petkovic and Coco Vandeweghe, often opt for one type of serve on the first two points and another on the next two. It’s tough to see much in the patterns among these players, especially since most of them are closer to the median level of predictability than they are to Wozniacki’s extreme consistency.
I included the final column, FSP(5+), to illustrate another aspect of Caro’s uniqueness. While she closely follows her script for the first four points, she reverts to almost 50/50 wide and T serves after that–even in the more extreme 2017-present subset of matches. Many of the other players on this list do not. Angelique Kerber, for instance, is a near Woz-level lock to go wide in the ad court late in games. She hits wide first serves more than 80% of the time at 40-30 or 30-40, and 73% of the time at AD-40 or 40-AD. Henin also stuck with her preferences on higher-leverage points.
Equilibrium
For whatever reason, Wozniacki is comfortable with this pattern, and is confident that it works. Or, at least, that it doesn’t work against her. It’s not a secret–the sequence came to my attention after Siegemund’s coach pointed it out during an on-court coaching visit in Charleston.
Tennis is full of decisions like this: when to follow a pattern, and how often to vary things to keep an opponent from getting too comfortable. On this week’s podcast, Carl and I speculated about how often a player would need to deploy an underarm serve in order to force a returner out of position. If Wozniacki’s tendencies are any indication, the answer is: not very often. The mere fact that Caro could serve the other direction was apparently enough to prevent Niculescu or Siegemund from pouncing on her first serves, even if Woz stuck to the script from the first game to the last.
I realize I’ve left a lot of questions unanswered. Does Caro win more first serve points when she varies her delivery more? Does she follow any similar patterns with her second serve? Does she use the results of the first four points to help decide the direction of the following points? Are there particular types of players who force her to mix things up–as Madison Keys did in the Charleston final, with her aggressive return tactics?
Keep an eye on this space–maybe I’ll be able to offer some answers. In the meantime, I hope you derive some extra enjoyment the next time you watch a Wozniacki match, knowing in advance where her next serve will go. Or, perhaps, you’ll witness one of the rare occasions when the most predictable woman in tennis goes off-script.
Thanks to Kees for charting the Siegemund match, passing along the on-court coaching conversation, and providing the impetus for this post.