I think part of it is just the length of the match, having nothing to do with TV viewers. The last 4 GS matches between Nadal and Djokovic were 3:50, 4:10, 4:40 and 5:50. So if you start the match at 7, you're probably counting on finishing at 11:30, possibly later.
But from a programming standpoint, they probably realize they're not going to compete with the NFL or regular prime time programming for the whole night. I think they'd rather have the first half of the match early and the last half of the match (where viewers are more likely to watch) in prime time - than have the first couple sets in prime time but the finish after all the east coast viewers have gone to bed.
JMO