Well, on TV it didn't look like cramps. He couldn't put in a first serve reliably and oftentimes the speed was in the low 90s.
He couldn't pronate on his second serve, therefore they kept going long and he wound up with double digit double faults, all except one of them coming in the last two sets. It really looked like an arm problem to me. It really could be any number of muscles or tendons, but I would guess it was plain old tennis elbow
developing in compensation for an underlying shoulder problem. I would guess that the shoulder problem emanates from some trouble with the biceps tendon(s) insertion (same injury Rafter had, not rotator cuff)...which one I don't know...seeing as how extra pronation on the forearm can compensate for what is usually generated with the biceps with the full service motion. Purely guessing though.
Anyway, I hope there are no tears...if there are they are keeping quiet about it or they are too small to see on MRI.
(FYI: in case anyone wants an anatomy lesson, the rotator cuff is made up of four muscles: supraspinatus, infraspinatus, subscapularis and teres minor. However, many of the upper arm muscles have tendons inserting in on or around the shoulder joint, and their injury can affect motion just the same. Some examples are the biceps long and short heads and the pectoralis minor and the deltoids.