In US-Open 2015 mixed doubles final I saw tie-break instead of the third set.
What is that ?
Is it applied only in mixed doubles?
When this rule is applied in tennis?
Is it applicable for all tournaments?
First of all, Mixed Doubles competition is played only in the 4 Grand Slams. There is no mixed doubles event except in Olympics which falls outside the ITF. The rule came into effect in 2001(I think). It is known as Super Tiebreak. Before that, mixed doubles competition used to play best of 3 set matches.
If the teams have won 1 set each, instead of playing the deciding 3rd set, they play a super tiebreak of 10 points, ie. first to reach 10 points with a lead of 2 points wins the 3rd set & hence the match.
The rule was brought in to speed up the Mixed doubles competition & make it more attractive
What: if you are talking about "Super Tie-break" the appendix V of ITF rules of tennis you can see info about Match tie-breaks (10 points): this is a tie-break to 10 points, a shorter solutions played instead of third (final) set
When: suggested by ITF in 2001, experimental in 2002, it becames a rule from the year 2003 source
Applied: it is applied also to double matches (men and women) and also in other tournaments out of Gran Slam circuit: