And in practice, Hawk-Eye has actually made tennis a far more sportsmanlike game than it was in the past. The players, once under the thumb of the infallible officials, can now both question authorities and definitively prove it when they're wrong—which has curbed some of the on-court fits that became a hallmark of tennis in the Open era. Had the computer-assisted challenge system been around in John McEnroe's time, for example, he may never have uttered the "You cannot be serious!" heard 'round the world (and he's said as much himself).

Dr. Allen Fox, Ph.D., a pro tennis player-turned-sports psychologist and the author of Tennis: Winning the Mental Match, remembers watching Nicolas Kiefer, a talented, volatile German player who peaked at No. 4 in the world rankings in 2000, play a match just after the advent of Hawk-Eye. Kiefer, as Fox puts it, "always felt like he was getting cheated," and in this particular moment, he found himself on the unlucky end of what he thought was a bad call.

"He'd get all emotionally unraveled during his matches. The whole world was conspiring against him, and so forth," Fox says. "So he was starting to go into his histrionics act when he realized he could challenge it."

According to Fox, Kiefer challenged the call, and it turned out the line judge's ruling had been correct. "It sort of took all the wind out of his sails," Fox recalls. "I could almost see the wheels turning, and for a minute he didn't really know what to do."

It's players like Kiefer, Fox says, who most benefit from challenge-aided tennis—those players who take the line judges' calls personally, lose their composure, make excuses, or dwell on unfavorable points after they happen, or whose level of play drops when they're flustered. "It evens them out right away," Fox says. "The strong-headed [players], they straighten themselves out anyway." In other words, Hawk-Eye can help even the playing field by lessening the overemotional reactions that sometimes causes less resilient players' concentration to capsize.

Even the most resilient players, though, benefit from being able to challenge. Fox, a onetime NCAA tennis champion and Wimbledon quarterfinalist, says that no matter who you are, "It takes an effort of will to put [a perceived bad call] out of your mind. When you've actually won a point, but then through somebody's error you lose it, that's really hard to accept."

So Hawk-Eye helps everybody, according to Fox. "But," he adds, "it can keep the bad-headed guy in there longer."

That's not to say, though, that Hawk-Eye has fully eliminated tennis' quarrel-prone nature. Tantrums in the instant-review era are alive and well for plenty of other reasons—and player challenges have been known to create controversies of their own. There are still disputes over how quickly a challenge must be requested after a point has ended, for instance, and at the 2007 Wimbledon singles final, the normally imperturbable Roger Federer was so ruffled by some seemingly unbelievable Hawk-Eye rulings that he swore on live television and then insisted that the shot spotter be turned off for the rest of the match. And though Hawk-Eye is unquestionably more precise than the naked human eye, it, like all machines, has imperfections. It's advertised as having a 3.6-millimeter average margin of error, and, as the Czech power hitter Tomáš Berdych was dismayed to learn in 2009, its performance can be affected by shadows on the court.