Massu is a real gutsy player who has been a bit off form lately. last year I made stacks on him by such results as him dumping Roddick out in Madrid at +700 for the match.
My feeling is that there was a lot of mug betting on Dent & Fish from folk who normally do not bet tennis, and this was reflected in the prices being posted on these 2 guys.
In general, there is good money to be made by opposing US players at suitable moments, e.g. I was on Muller to beat Agassi at 7 to 1 or so in the Legg Mason.
At 6 or 7 to 1, you only need to win about 1 in 4 or 5 times on average to make decent returns.