Tennis’s scoring system has long been known for its quirkiness. Your first two points are each worth 15, but not your third. You need to win two sets to triumph in most matches—except in the men's singles at the four “grand slams”, which are best-of-five. These sets are typically decided by complex tiebreaks—but not in the deciding sets at Wimbledon, the Australian Open and the French Open. And every unit of a match is completed only when one player builds a two-point advantage.

The “win-by-two” nature of tennis is a big part of what makes it exciting, since it prolongs the climax of each contest. Yet it has increasingly come under fire as the sport aims to become more media-friendly by shortening its matches. A decade ago, two rule changes—a winner-takes-all point at deuce and a ten-point “super-tiebreak” instead of a deciding third set—were introduced in second-tier doubles competitions, with the aim of abbreviating them.

You could be forgiven for missing the news: doubles tennis is significantly less popular than the singles variety, and the tweaks did not apply to the men’s or women’s doubles tournaments in grand slams (though they have been used in mixed doubles at the Australian, French and US Opens). But fans might want to start brushing up on the alternative rules. Steve Simon, the chief executive of the Women’s Tennis Association, has argued that the new system should be adopted more broadly; Novak Djokovic, one of the stars of the men’s game, has said that it needs to “improve and evolve”.

The two changes are designed to compress crucial moments. The winner-takes-all point at deuce, known as “no-ad” scoring (short for “no-advantage”), limits each game to seven points. The third-set super-tiebreak, meanwhile, is a slightly longer version of a conventional seven-point tiebreak. Players still need to win by two points, and sometimes contest as many as 30. But this process is shorter than enduring an entire extra set, making marathon, broadcast-unfriendly tussles less likely.

A super-tiebreak reduces the length of a typical three-setter by 20% and almost eliminates the possibility that a match will exceed two hours. Replacing the win-by-two rule with deciding points at deuce knocks at least another five minutes off the length of a typical fixture. Last week’s ATP World Tour Finals demonstrated the effectiveness of both adjustments. While Andy Murray played two singles matches that each lasted more than three hours, none of the doubles ties lasted more than one hour and forty minutes. Barely a quarter of them exceeded one hour and twenty.

What about excitement: does the decisive importance of the final point in the no-ad rules make up for the epic back-and-forth of the deuce system? Is a super-tiebreak shoot-out more gripping than a draining third set?

A metric called—appropriately enough—the “excitement index” (EI) can help here. Originally created for American football and easily adapted for tennis, it measures the importance of each play and then aggregates these figures into a single number. In tennis, each point is rated by its volatility: if winning a rally increases your probability of victory by 5% and losing the same exchange reduces your chances by 5%, then that moment has a potential 10% swing, or an EI rating of 0.1. The EI of a match is the average of these volatilities across all points, multiplied by 1000. It ranges from about 10 (a pretty dull contest, where the average point has a 1% impact on the final result) to 100 (a riveting one, where the typical rally comes with a 10% swing). Mr Murray’s heroic three-hour semi-final victory over Milos Raonic on November 19th was near the top of the scale, at 94.

The new rules produce a few very volatile moments, such as deciding points in no-ad games, at the expense of a larger number of quite important exchanges. This has little impact on the excitement levels of most contests. The singles matches at last month’s ATP Shanghai Masters are a good example. Played using the traditional rules, these had an average EI of 50.5. Computer simulations of the same fixtures using the new formats showed an average EI of 50.8.

But at the extremes, the alternative rules curtail the tension. The Shanghai matches that went to three sets had an average EI of 65.1, while simulations of the same pairings averaged just 56.7. When the computer replicated the duel between Mr Murray and Mr Raonic 100 times, using no-ad games and super-tiebreaks, it did not exceed an EI of 94 once. Of course, if those two were to play each other several more times, they would not always produce such a memorable contest. The last time they did so, in Cincinnati in August, Mr Murray breezed through in straight sets, requiring only 90 minutes. Still, these topsy-turvy, high-EI matches occur several times in a typical tournament, and the alternative rules would greatly reduce their number.

There is one effect of the modifications that might compensate for this lost excitement: the shorter the format, the higher the odds of an upset. The top-seeded team in last week’s doubles event, the French pairing of Pierre-Hugues Herbert and Nicolas Mahut, lost all three of their matches, with two of their defeats occurring in a super-tiebreak. Given a match-up in which the favorite has a 65% of winning under traditional rules, his or her chances fall to 62% with the alternative format.

Broadcasters are not the only ones who might benefit from briefer contests. In an increasingly physical sport, shorter matches might reduce injuries and keep superstars on the court. Less onerous grand-slam doubles tournaments could attract more high-profile singles stars to enter them, too. Still, the importance of the extremes—and their unpredictability—can’t be overstated. The most memorable matches, like the famed 2008 Wimbledon final between Roger Federer and Rafael Nadal and the 11-hour battle between John Isner and Mr Mahut at the same tournament in 2010, derive their drama in part from their length. Doubles tournaments have rarely drawn much attention, so tweaking their format is harmless enough. But in singles, it turns out that the quirky scoring system plays a crucial role in making the best matches as exciting as they are.