It is called a match.

If you assume you have a 50% chance of winning.   

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 than .5 then plug in any number you want. 

Up 2-1 in 5 set match is the same math.