In tennis, the number of sets you need to win to secure a match depends on the format being played:

• Best-of-three sets: You need to win 2 sets to win the match. This is the most common format used in professional tennis, including most women's matches and many men's matches outside of Grand Slams

• Best-of-five sets: You need to win 3 sets to win the match. This format is typically used in men's singles matches at Grand Slam tournaments like Wimbledon, the US Open, the French Open, and the Australian Open

Each set is usually won by the first player to win 6 games with a margin of at least 2 games. If the set reaches 6-6, a tiebreak game is played to decide the set

Summary:

Format| Sets Needed to Win
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Best-of-three| 2 sets
Best-of-five| 3 sets

This means most matches are decided by winning two sets, except for men's Grand Slam matches where three sets are required for victory