From 1970 through 1974, what tiebreak format did the US Open use?

From 1970 through 1974, the US Open used a best-of-nine-point sudden-death tiebreaker.

The format was introduced when the US Open became the first Grand Slam tournament to use a tiebreak to decide a set tied at 6–6 in games. A player won the tiebreak by reaching five points first, rather than needing a two-point margin. If the score reached 4–4, the next point decided the set, which explains the name “sudden death.”

The system was associated with Jimmy Van Alen's experimental scoring ideas. Serves alternated in a distinctive 2-2-2-3 sequence, unlike the modern pattern. It was used in both singles and doubles at the US Open during this period.

The tournament changed to the International Tennis Federation's best-of-twelve-point system in 1975. That system generally required seven points and a two-point lead, so it was longer and less abrupt. The nine-point format is often confused with today's seven-point tiebreak or with modern 10-point match tiebreaks.