Guillermo Vilas (July-Sept. 1977)
The Argentine holds the longest streak, winning seven straight tournaments in 1977, including the U.S. Open.
Both Vilas' 46-match winning streak and his 53-match streak on clay ended that October, controversially. Vilas lost the first two sets to Ilie Nastase, who was using a novel racket with strings that did not intersect, the so-called "spaghetti-string" racket that created unusual spin. Vilas quit in protest of the racket rather than play out an expected loss.
Vilas won 28 straight matches after that, and the spaghetti-string racket was banned the next year.