It is called a match.
If you assume you have a 50% chance of winning each set.
For up 1-0 in a 3 sets
Take 1 - chance of losing the rest
1 - (.5 x .5) = .75
If you think they are better or worse than .5 then plug in any number you want.
Up 2-1 in 5 set match is the same math.