help with a tie-breaker question
I am a hoping someone can help settle a dispute regarding tiebreakers in 8-game & 10-game pro-sets.
One person says a tie breaker occurs at 8-8 & 10-10 (8 game & 10 game respectively)
Another says it occurs at 7-7 & 9-9
The former says that an 8-game set = 8 games. So until at least one player has 8 wins, the match continues. In theory, an 8-game match could end with the score 9-7 (having gone to 8-7, then 9-7 to win by two) without ever going to tiebreaker.
The later agrees 8 games is the goal, but argues that since you have to win by two, a 7-7 score insures that neither player can have 8 and win by two. At this point the tie-breaker is played to win the final deciding game 8-7 (with tie-breaker score). A 9-7 final under this scenario would be impossible.
Which is correct? Does it depend on the tournament?
Can anyone shed light on this situation?