You guys dont understand: Del Potro is injured, but one fact is certain: He never was a great returner of serve. If you look at the matches he wins, when his opponent has a high first serve percentage he does have problems. With that said, on a good serving day, no one can win against Karlovic, its as simple as that.
What I think- Karlovic in 5. Something like 4-6 7-6 7-6 4-6 8-6.
I heard he is healthy. Yeah, he isn't too big returner but not bad and Karlovic on clay...
Del Potro is very good in tie-breaks, he beaten Isner 7-6 and 7-6 at Indian Wells too. He has good enough serve for this type of guys. Karlovic can't break him.
It is just because Karlovic play totally outform recently, he lost his last two matches in row.