In a best-of-five-set tennis match, a player must win three sets.
“Best of five” means the first player to win a majority of the five possible sets wins the match. Since a majority of five is three, the match ends as soon as one player reaches three set victories. The score can therefore be 3–0, 3–1 or 3–2.
Men’s singles matches at the four Grand Slam tournaments traditionally use the best-of-five format. Most women’s singles matches and nearly all regular tour matches use best of three, where two sets are required to win.
The phrase does not mean a player must always play all five sets. A straight-sets victory ends after three sets, while a 3–2 match uses the maximum five. Tie-break rules decide many individual sets, but they do not change the number of sets needed to win the match.