Guillermo Vilas (born 17 August 1952) is an Argentine former professional tennis player. Vilas was the No. 1 of the Grand Prix seasons in 1974, 1975 and 1977. He won four Grand Slam titles, the year-end championship, and 62 ATP titles overall. World Tennis, Agence France-Presse and Livre d'or du tennis 1977 (Christian Collin-Bernard Ficot), among other rankings and publications, rated him as world No. 1 in 1977. In the ATP computer rankings, he peaked at No. 2 in April 1975, a position he held for a total of 83 weeks, although some have argued that Vilas should have been ranked No. 1 for at least 10 weeks, particularly in 1977 when he won 2 grand slams. He was inducted into the International Tennis Hall of Fame in 1991, two years after his first retirement.