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FiveThirtyEight

Roger Federer has a good chance later this year to pass Pete Sampras for most aces in his career. Ivo Karlovic, in turn, is hot on Federer’s heels for No. 4 on the all-time men’s tennis aces list and could pass him at the U.S. Open this week.

You may start hearing more about this leaderboard as Federer and Karlovic keep overtaking former greats and each other. If you do, it’s worth keeping in mind that among trivial sports records, the all-time aces title is a particularly meaningless accomplishment:

  • The ace counts only go back to 1991, when umpires at ATP World Tour and Grand Slam events started recording point-by-point data, including aces (serves that land in and which the returner can’t touch with his racket). All-time leader Goran Ivanisevic and Sampras (No. 3 in aces) debuted three years before the match stats did. Sampras probably would have a much wider lead over Federer and No. 5 Karlovic if his 125 matches from his first three years on tour counted toward his ace total. And Ivanisevic would have a more secure grip on No. 1 if he got credit for his 136 matches between 1988 and 1990; as it stands now, his countryman Karlovic could pass him as soon as late next year. No. 11 Marc Rosset — who also debuted in 1988, a banner year for big servers — might have held the lead among Swiss players over Federer for longer if his 62 matches through 1990 counted. Even more of the careers of big servers Boris Becker, John McEnroe and Roscoe Tanner are shrouded by the sport’s statistical blind spot.
  • The Davis Cup, the sport’s international team competition, counts toward players’ official match records. But merely for administrative reasons — “with all the ties all over the world we don’t have a system to get the data,” ATP stats overseer Greg Sharko said in an email — Davis Cup stats don’t count toward ATP totals. That means Karlovic doesn’t get credit for his 78 aces in a Davis Cup loss in 2009. On the other hand, Karlovic has played only 17 Davis Cup singles matches, while Federer has played 42 — more than Ivanisevic and Sampras, though three fewer than the 45 played by Andy Roddick, who is second on the all-time aces list. No aces from those matches count toward these players’ totals.
  • All counting stats are crude ways to estimate athletes’ ability. Some quarterbacks amass more yards because they play for pass-happy offensive coordinators. Baseball players can get more runs batted in by batting often with teammates on base. Raw ace counts in a single match face the same problem: Karlovic got those 78 aces in the 2009 Davis Cup match because it went to 16-14 in the fifth set. That’s less impressive than his 44 aces in a three-set match earlier this year. It’s all the less meaningful, then, to compare ace counts over a season or career. Federer has a narrow lead over Karlovic mainly because he has played more than twice as many matches. That’s a credit to Federer’s overall superiority as a tennis player compared to Karlovic, and to just about everyone else who ever has lifted a racket. The better you are at the sport, the more you win, the more matches you play, and the more chances you have to rack up aces. It’s hard for Karlovic to keep up with Federer while losing in the first or second round of many tournaments and watching his rival get to keep serving for four to six matches into the final. But if the point of ace counts is to say who’s the best player, there are plenty of better ways to measure it. And if it’s to measure who has the best serve — ignoring returns and everything else in tennis — then a rate stat would do much better. Karlovic has aced opponents on nearly 23 percent of his service points since 2010, or more than double Federer’s 10 percent. Appropriately, Karlovic hit 24 aces in his U.S. Open debut Tuesday, while Federer hit 10. Both men won.
  • Karlovic has started catching up — by playing more matches. The hard way to do that is to win more matches at big tournaments. The easier way to do it is to sign up for lots of the lower-level tournaments called 250s, which are the weakest ATP World Tour events that count toward ace counts. Karlovic has played in 23 250-level tournaments over the past two years. Federer has played in four. Those events have helped Karlovic make up ground on Federer, both because he’s getting more matches and because he’s playing weaker opponents. By median and mean ranking of opponents, Karlovic’s schedule this year and last has been roughly twice as easy as Federer’s. Weaker opponents are, on average, easier to ace. For instance, the last time Federer and Karlovic played each other, Federer’s ace percentage was higher than usual, at 11.5 percent, while Karlovic’s was lower, at 19.1 percent.

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