Some worried that instant replay would slow down the game. Others complained that players would manipulate it to anger opponents. Others simply deemed it unnecessary. Through the first few rounds at the Nasdaq-100 Open in Key Biscayne, Fla., however, naysayers have had little evidence to justify their concerns.

Yes, it’s early, but so far the introduction of instant replay in tennis has been a resounding success. Players, who are permitted two challenges per set (plus a third if there is a tiebreaker),are questioning calls, but not too often. Umpires and linesman have been proved correct most of the time, though incorrect enough times to show how difficult their jobs have become and how useful this technology can be.

Replays have been both instantaneous and decisive – on at least one occasion this weekend, the result of a challenge appeared before television commentators could even explain who was challenging what. And fans have thoroughly enjoyed the brief moment of suspense between the instant a challenge is made and its resolution, leading either to a winning smile or a sheepish grin signaling defeat.

Even Roger Federer, the world’s top player and a staunch opponent of what he considers an unwarranted technological advance, has gotten in on the action. During the first set of his second-round match this weekend against Arnaud Clement, Federer challenged a Clement serve that had been scored an ace. Replay showed the ball wide – instead of 30-30, Clement had to hit a second serve at 15-30. Federer won the point and the game, the last of the set.

After nearly two rounds of tennis, data from this experiment clearly revealed that instant replay is a necessary addition to the game. As of Saturday afternoon, players had made 53 challenges and won 16 of them, a success rate of 30%. None of the challenges occurred at particularly tense moments or played a substantial role in the outcome, and ESPN’s television commentators were quick to point out how often the umpires were correct. It’s more relevant, though, to note how often they were wrong: Nearly one out of every three difficult calls.

Considering how quickly tennis balls move and how much they spin these days, a 30% error rate on close calls is not bad. But in an absolute sense, it’s not terribly good, either. There’s no need to make so many mistakes in umpiring a fast-paced sport when the technology to prevent it is readily available.

Will instant replay change the way matches are called? After one weekend, it already seems likely that it will, and for the better. It might even affect calls at tournaments and matches where the technology will not be used (it’s only appearing at a few hard-court tournaments this summer, including the U.S. Open, and only on show courts).

Take an overrule during Saturday evening’s match between Maria Sharapova against Na Li of China. When Li slammed a backhand down the line, chair umpire Lynn Welch immediately declared, “Out,” after the linesman made no call. Li appealed and won – her backhand had clipped parts of both lines. The point is not that Welch was incorrect; it’s that her call was an exceedingly difficult one to make. When a ball is traveling at 70 mph and lands near a line 30 feet away, it’s not wise to believe that you can call it in or out with any real certainty.

Umpiring is no easy job, and umpires and linesman have always behaved – out of necessity – as if they were absolutely certain about the calls they made, even if the laws of physics demanded less confidence. At times, this has morphed into overconfidence, especially when chair umpires overrule close calls that occur too far away from the chair. If instant replay makes umpires more timid in this regard, the sport will be better off.

Better still, umpires might champion instant replay once they realize that it does not belittle their work or make them unnecessary, but aids them in doing a better job at what they already do reasonably well. If that happens, the day when umpires are allowed to turn to replays of their own accord, rather than waiting for a player to protest, will follow.

***

AMERICAN WOMAN? The Nasdaq-100 looked to be a terrible tournament for American women until Meghann Shaughnessy and Jill Craybas shook up the draw by upsetting Justine Henin-Hardenne and Kim Clijsters, respectively. There’s little hope, unfortunately, that either of these veterans will survive much longer (Shaughnessy lost yesterday), and the prospects for American fans for the remainder of 2006 are grim.

Both Serena and Venus Williams withdrew from this tournament with injuries; Serena, who has played three matches this season, has slipped to no.61 in the rankings – lower than Craybas. Lindsay Davenport’s ailing back forced her out of this tournament and her ranking will likely fall further during the clay court season. The next-highest ranked American is Lisa Raymond at no. 71.

***

WHO’S THAT BRIT? Tim Henman is having his best tournament in more than a year. Henman defeated Marat Safin, who recently returned from a knee injury, and Lleyton Hewitt, who had defeated Henman in their previous eight meetings.

***

ANDRE AND OUT Andre Agassi missed this tournament for the first time since 1987, pulling out with a recurrence of the back injury that sidelined him last year. He plans to skip the upcoming clay-court season and likely will not appear in another tour event until June. Agassi, who turns 36 next month, also admitted that his career might be over if his back does not improve. He still hopes to play Wimbledon.

tperrotta@nysun.com