The U.S. Open is a prestigious Grand Slam tournament. Tennis is a game played between two players (singles) or between two teams of two players (doubles). Players use a stringed racquet to strike a hollow rubber ball covered with felt (most of the time yellow, but can be any color or even two-tone) over a net into the opponent's court. Originating in Europe in the late 19th century as "lawn tennis", with its roots going back to the ancient game of real tennis, tennis spread first throughout the English-speaking world, particularly among the upper classes. Tennis is now once again an Olympic sport and is played at all levels of society, by all ages, and in many countries around the world. Except for the adoption of the tie-breaker in the 1970s, its rules have remained remarkably unchanged since the 1890s. A recent addition to tennis has been the adoption of "instant replay" technology coupled with a point challenge system which allows a player to challenge the official call of a point. Along with its millions of players, millions of people world-wide follow tennis as a spectator sport, especially the four Grand Slam tournaments: Australian Open, French Open, Wimbledon, and the U.S. Open. The growth of tennis in Eastern Europe and the Far East has been especially notable in recent years.