Ivo Karlovic’s Survival and the Key to Aging in Men’s Tennis

Italian translation at settesei.it

Let’s just get this out of the way first: Ivo Karlovic is amazing. The Croatian didn’t play his first tour-level match until he was 22 years old, and he didn’t crack the top 100 for two years after that. Yet he eventually reached No. 14 in the world, won over 350 career matches, and claimed nine tour-level titles. Now, a few weeks shy of his 40th birthday, he’s coming off an ATP final in Pune, where he came within two points of ousting top-ten stalwart Kevin Anderson and ensured that he’ll remain in the top 100 through his milestone birthday next month.

The fact that Karlovic is one of the tallest men ever to play the game and that he holds a wide array of ace records is beside the point. (Though it’s certainly worthy of discussion, and I hope to dive into aging patterns and playing styles in a future post.) Yes, his first-strike brand of tennis, avoiding the bruising rallies that have worn down the likes of David Ferrer, may make it easier to compete at an advanced age. On the other hand, he remains of the few men on tour to regularly serve-and-volley, a tactic that scores of younger, quicker men can’t execute effectively. He is, quite simply, one of a kind.

Despite his uniqueness, Karlovic represents an important aspect of men’s tennis in the 2010s. The ATP has gotten older since he broke in almost two decades ago, and ten men aged at least 33 are ranked higher than the Croatian. One of them, 37-year-old Roger Federer, remains one of the best players in the game. The average age of elite men’s tennis may be creeping back down, but it is still the golden age of 30-somethings.

Men like Karlovic and Federer have seemed to defy the usual logic of aging. Most sports have a reliable “peak age” at which players can be expected to to perform their best. Up to that point, competitors are developing both physically and mentally; after the peak age, physical deterioration sets in and performance declines. There’s always plenty of variation around the average, but the overall trajectory–break in, rise, peak, fall, retire–is predictable enough.

In part, Karlovic has followed that path, just with a late start and a surprise second peak in his 30s. To compare year-to-year performances, I calculated each player’s dominance ratio (DR), a useful measure of overall performance calculated as the ratio of return points won to opponents’ return points won, and adjusted it for quality of competition. (The adjustment algorithm gets complicated; I first outlined how it controls for each player’s mix of opponents here.) 1.0 is average, and the typical range runs from about 0.8 (soon to head back to challengers) to 1.2 (big four territory). The following graph shows Ivo’s DR at each age, along with a smoother three-year moving average:

Karlovic hit his primary peak around age 31, a bit late but not entirely atypical for the era. Even if we ignore the surprise spike at age 36, he remained an average player (roughly speaking, a card-carrying member of the top 50) until age 35. In 2017 and 2018, we finally witnessed a downward trend, but if Ivo’s feat in Pune is any indication, he might be turning things around once again.

Nearly every professional tennis player retires before they reach Karlovic’s current age, so we’ll never know what bonus peaks we missed. Of course, many of those retirement decisions are due to injury, so at least some of the Croatian’s late-career success must be credited to his ability to stay healthy enough to soldier on. Let’s look at an even more baffling aging pattern, one that belongs to a player who will almost definitely retire before seeing the kind of late-career decline that Karlovic experienced in 2017 and 2018. Here’s Federer:

By the measure of competition-adjusted DR, Federer’s best season came at age 34. Even if you don’t buy that, the overall trend is clear. He continues to play at or near his peak, past the age at which his peers become Davis Cup captains and have Tour Finals round-robin groups named after them.

Federer has been able to stay off the injured list for almost all of his 20 years on tour, and health–the simple fact of showing up for most tournaments–may be the most underrated skill in men’s tennis. The vast majority of players who don’t survive to post elite seasons in their mid- and late-30s aren’t slowly drifting down the ranking list, like a baseball player who plays every game in his 20s, then moves into more and more limited part time roles as he ages. Instead, they drop out, perhaps because of a single career-ending injury, the general accumulation of nagging problems, or lack of desire to wholeheartedly pursue the sport at the expense of everything else.

The following graph shows the two ways in which players fail to maintain their previous level from one year to the next: Playing worse tennis (measured by competition-adjusted DR), or leaving the tour. The latter is defined by contesting fewer than 20 tour-level matches, something that any reasonably healthy player with a ranking in the top 100 should be able to manage. At every age, players drop out at a surprising clip, and that rate begins to overtake the percentage of players who stay on tour but perform at a weaker level around the late 20s:

The “Leave tour” rates slightly overstate the number of disappearing players, since about one-quarter of them eventually return to the tour, like Andy Murray is trying to do in 2019. But even accounting for the number of comebacks, a hefty share of the players we expect to steadily decline are either forced off tour by injury or choose not to continue.

Selection bias

All of these disappearing players make it extremely difficult to construct an aging curve for men’s tennis. One common approach to measuring such a trajectory is to identify all the players who competed in consecutive seasons (say, their age-25 and age-26 campaigns), figure out how much better or worse they performed in the latter year, and average the differences. When we do that for ATP players born since 1970, the results are downright bizarre. The worst year-to-year change is from age 21 to age 22, when DR decreases by about 2.3%, even though we would expect youngsters to be developing their game for the better. The strongest year-to-year change is from age 30 to age 31, with an improvement of 4.0%, when we would expect a plateau or even a slight decline.

Because these ratios don’t include the players who drop out, most of the year-to-year ratios reflect an improvement:

Age       Year-to-year DR ratio  
19 to 20                  -1.7%  
20 to 21                  +0.9%  
21 to 22                  -2.2%  
22 to 23                  -0.3%  
23 to 24                  +1.5%  
24 to 25                  +1.1%  
25 to 26                  +0.7%  
26 to 27                  +1.5%  
27 to 28                  +1.2%  
28 to 29                  +3.5%  
29 to 30                  -0.8%  
30 to 31                  +4.0%  
31 to 32                  +2.6%  
32 to 33                  +0.7%  
33 to 34                  -0.5%  
34 to 35                  +3.0%  
35 to 36                  -0.4%

If we assembled these ratios into an aging curve, we’d see a line staggering upwards, as if we could expect players to continue improving for as long as they cared to compete.

However, things start to make sense when we acknowledge the selection bias and reframe our findings accordingly. It isn’t true to say that the average player steadily improves forever. But it is more believable to say this: The average player who remains healthy enough to play a full season and has the desire to compete full-time can expect to improve well into his 30s. The older a player gets, the less likely that the second claim applies to him.

As they say, half of success is just showing up. By age 39, most pro tennis players have long since started showing up somewhere else. By dint of sheer perserverance, a bit of luck, and one of the most dominant serves the world has ever seen, Karlovic has shown that tennis’s aging curve is even more flexible that we thought.