Instant replay has arrived in big-time professional tennis - on the show courts, that is… The U.S. Open became the first Grand Slam tournament to embrace the electronic line-calling technology produced by Hawkeye Innovations from the UK.  The system was used during the U.S. Open Series’ tournaments leading up to the U.S. Open and received very favorable reviews.  Surprisingly, the players’ challenges on over 800 line calls, averaging 2-3 challenges per match, were only right about a third of the time!  Like other new technologies introduced to tennis in the past, e.g. Cyclops electronic service line calling system, the instant replay system is primarily for TV.   It is only being used on Arthur Ashe stadium and in the Louis Armstrong stadium, and therefore, tends to benefit the top players much more than everyone else.  Instant replay is a great application of technology to pro tennis, but is it really fair to play part of the tournament under one set of rules and the rest of the matches under another set?  What do you think?