Sunday, April 13, 2014

Myth vs. Math: Djokovic Return

This is the second installment in a five-part series called Myth vs. Math. In this series, I am going to take a look at five widely-accepted statements that tennis writers, analysts, fans, and commentators frequently make. I'm going to take these statements and see how they hold up against the numbers. The first statement in this series was "big servers have a notable advantage in tiebreakers." Now I am going to look at the statement that "Novak Djokovic has the best defensive return in the game right now."

The Myth

Over the past three and a half years, Serbian Novak Djokovic has been arguably the best player on tour. Since the start of 2011, Djokovic has won 25 titles, including five grand slams, and has been ranked No. 1 in the world for over 100 weeks of that time.  Djokovic has also gone 21-10 against his main rivals, Roger Federer and Rafael Nadal, in that span.

Along with his backhand and movement, the part of Djokovic's game that gets credited for his success has been his return game. In his career, Djokovic has won 32% of his return games, which reached a peak in 2011, when he was winning 39% of his return games. This has led to Djokovic drawing comparisons to the greatest of all time.

While many tennis experts won't claim that Djokovic is the greatest returner of all time, they will say things like "Djokovic has the greatest defensive return in the history of the sport, but Andre Agassi had a better aggressive return." The claim that Djokovic has the best defensive return in the history of the sport is widely accepted.

I narrowed the myth to just the greatest defensive returner among active players, so that the data will have a more conclusive result.  I define defensive returner to mean that the player is able to get into a rally after the return on any serve. So is Djokovic really the best player in the game at the defensive return?

The Math

The first stat I looked at is aces against. Djokovic is often compared to a goalie in hockey with the way he returns, because he is able to get his racket on any serve. So I looked at how much he has been aced in his 18 matches so far this season and how tough his opponents serves were.

In the Serb's 216 return games so far in 2014, he was aced 83 times, which comes out to .384 aces per return game. However, I also want to look at how tough his opponents were, so I calculated the aces per service game of each of the servers in 2014. Based on those numbers, I figured out how often they would have hit an ace, if those 216 games were played against an average returner. If Djokovic were an average returner, he would have been aced 124.6 times in those 216 games.

Therefore, Djokovic defended over 40 aces that an average returner would have been aced on. He allowed .384 aces per game in his matches compared to .577 aces by the average returner.

 This graph compares how many aces a player would hit in 1000 games against any opponent (orange) in 2014 and how many aces they would hit in 1000 games against Djokovic (purple). Against most players, Djokovic was much harder to ace than their average opponent, especially for Julien Benneteau and John Isner. However, for Stanislas Wawrinka and Marin Cilic, Djokovic was actually easier to ace than their average opponents. So can Djokovic really be the best defensive returner in the game if he is easier than average to ace for two of the opponents in his 18 matches?

The second stat looks at the more complete picture of defensive returning, but is not as directly associated to the return. I wanted to look at how many points Djokovic wins on his first serve return compared to his second serve return. This stat has a lot of other variables besides just the return, but it could give some insight into how well Djokovic returns the first serve.

The defensive return is more important on the first serve return than the second serve return. The goal of a first serve return is normally to get to a neutral point in the rally. However, when returning the second serve, the returner is usually already at neutral in the rally. So someone with a good defensive return would be able to get to neutral in the rallies quickly off the first serve, resulting in a nearly equal win percentage on both first and second serve returns.

This year, the world No. 2 has won 34% of points on his opponents first serve, which ranks seventh in the ATP. On his second serve return, Djokovic has won 55% of points, which ranks ninth, so the difference is 21%. Here's how that ranks among other return leaders on tour.

Name1st Serve Return2nd Serve ReturnDifference
Bautista Agut30%55%25%

Djokovic ranks seventh in difference between 1st serve return points won and second serve. While there are some surprises with Ramos, Young, and Sela all appearing near the top of the list, the majority of the list seems to make sense. It isn't surprising to think of Gael Monfils as a great defensive returner or Richard Gasquet as a particularly bad defensive returner.

Flaws in the Math
Neither stat looks at the whole picture. For the aces against stat, not being aced does not necessarily mean that the returns are any good or even going in at all. So not getting aced frequently does not necessarily make him a great defensive returner. With the second stat, a high difference could indicate that the player is very good at being aggressive on the second serve as well. A large difference does not guarantee that the returner is bad at getting returns in play.

However, when putting the two stats together, they each cover up the flaws of the other to give a more complete picture of the returners ability. The stats combine the ability of the player to make contact with the serve and the ability to get into the point off that return.

While Djokovic is by far an above average defensive returner, there are better defensive returners on tour who don't get the same attention as a six-time grand slam champion. Shots like the return Djokovic hit against Federer are replayed frequently on tennis broadcasts, which helps support the idea that Djokovic has the best defensive return. However, in reality, Djokovic has struggled against big servers like Isner. Even Andy Murray beat Djokovic with mostly just a good serve in the 2012 Dubai semifinals. Murray hit five aces and won 85% of points on his first serve, while winning just 25% on his second serve.

Meanwhile, Federer and Monfils have enjoyed great success in their careers against big servers. Even though Djokovic may make some acrobatic returns off the first serve, Monfils has had much more success doing it consistently. Federer, on the other hand, lacks the athletic returns that Monfils and Djokovic possess, but does an excellent job reading the serves of his opponents and being ready for them, resulting in a lot of balls put in play.

So even though Djokovic is one of the top defensive returners among active players, he isn't the best in the game right now.

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