While the official Association of Tennis Professionals (ATP) computer tennis rankings are used to seed players in tournaments, they are not used to predict a player's chance of winning. However, since the rankings are derived from a points rating, an estimate of each player's chance in a head to head contest can be made from the difference in the players' rating points. Using a year's tournament results, a logistic regression model was fitted to the ATP ratings, to estimate the chance of winning as a function of the difference in rating points. Once the draw for a tournament is available, the resultant probabilities can be used in a simulation to estimate each player's chance of victory. The method was applied to the 1998 Men's Wimbledon, 1998 Men's US Open and the 1999 Men's Australian tennis championships.