player of the 20th century by a panel of experts assembled by the Associated Press. Tennis writer Steve Flink, in his book The Greatest Tennis Matches of the Twentieth Century, named her as the best female player of the 20th century, directly followed by Martina Navratilova. Tennis magazine selected Martina Navratilova as the greatest female tennis player for the years 1965 through 2005. Tennis historian and journalist Bud Collins has called Navratilova "arguably, the greatest player of all time." Billie Jean King said about Navratilova in 2006, "She's the greatest singles, doubles and mixed doubles player who's ever lived." In 2018, a Tennis.com panel selected Serena Williams as the greatest female tennis player in the Open Era. In May 2020, the Tennis Channel ranked Williams as the greatest female tennis player of all time. In November 2018, Tennis.com polled its readers to choose the greatest women's tennis player of all time and Graf came in first. In July 2020, The Guardian polled its readers to determine the greatest female tennis player of the past 50 years, and Graf was the clear favorite, picking up nearly twice as many votes as any other player. Officials In most professional play and some amateur competition, there is an officiating head judge or chair umpire (usually referred to simply as the umpire), who sits in a raised chair to one side of the court. The umpire has absolute authority to make factual determinations. The umpire may be assisted by line judges, who determine whether the ball has landed within the required part of the court and who also call foot faults. There also may be a net judge who determines whether the ball has touched the net during service. The umpire has the right to overrule a line judge or a net judge if the umpire is sure that a clear mistake has been made. In past tournaments, line judges tasked with calling the serve were sometimes assisted by electronic sensors that beeped to indicate an out-of-bounds serve; one such system was called "Cyclops". Cyclops has since largely been replaced by the Hawk-Eye system. In professional tournaments using this system, players are allowed three unsuccessful appeals per set, plus one additional appeal in the tiebreak to challenge close line calls by means of an electronic review. The US Open, Miami Masters, US Open Series, and World Team Tennis started using this challenge system in