In January, tennis legends Roger Federer and Rafael Nadal tried out a souped-up version of the sport called Fast4. They played separate exhibitions in Australia under the new format, which has no lets (serves that go in after hitting the net count) and no-ad scoring (at deuce, the winner of the next point wins the game). Sets are played to four games rather than six (at 3-3, a shortened tiebreak ensues) and points within games are scored 1, 2, 3, 4 rather than 15, 30, 40, game, so those unfamiliar with tennis can follow along.

“It’s...