I was expecting a reply like this
Even you newbies are predictable
Believe it or not, Cilic used to be a fairly good player.
It's not a prediction, it's fact. Isner's level is above Cilic's by some way nowadays. So, if Isner plays well enough, he wins. Simple. Understand?
He still is a fairly good player, although not as good as he used to be. You can't be ranked as high as Cilic if you're a clown.
And no, it's not a fact. The fact that Isner has been playing better than Cilic lately doesn't mean that Marin can't play lights out for a match and take him out even if Isner plays great.
Tennis is full of cases where players catch fire and beat players who are theoretically better than them and higher ranked, especially when we're talking about players who are capable of playing superb tennis like Cilic. You'll soon learn that if you stick around.