What year was the National Tennis League established in the United States?

The National Tennis League (NTL) was formed by former U.S. Davis Cup captain George MacCall in 1967, as a governing body to an American professional tennis tour. MacCall signed several players to contracts, including Arthur Ashe, Rod Laver, Roy Emerson and Stan Smith. In 1970 it was sold to the World Championship Tennis (WCT), a competing professional tennis league run by Lamar Hunt.

The NTL followed in the wake of MacCall's departure from the existing International Professional Tennis Association (IPTA), which had been created by promoter Wallace Dill in 1966, and preceded the creation of World Championship Tennis (WCT), formed by Lamar Hunt and David Dixon. The NTL differed from the IPTA and from WCT in being the first to sign women to professional contracts in addition to men.

Billie Jean King became the first woman of the Open Era to sign a pro contract to tour, in a group with Rosie Casals, Françoise Dürr and Ann Haydon-Jones as the women's section of the National Tennis League. On April 1, 1968, they signed with the National Tennis League, as George MacCall's troupe was called. Frankie Durr and Rosie Casals received a guarantee of $20,000 per annum for two years, Ann Haydon-Jones was $25,000 and Billie Jean King's was $40,000. They joined the six men that he already controlled - Emerson, Laver, Gimeno, Gonzales, Rosewall and Stolle. A group of ten was an attractive proposition to offer and he believed he was going to do well with them.