0) first of all : do you have the latest build ? (build #9 from September 17th)
1) did you look the energy level of the day when playing against Nestor ?
Else, I checked and he wasn't set a total double specialist, I changed that (it'll be reflected in next update).
And is it possible to see a screenshot of your character's skills ?
2) how high is your injury risk level ?
3) see 0) ...
4) strange... anyone else experienced this ?
5) maybe coz your player isn't good enough in rally ? So he can break Sampras when he does a poor serving game, but not Chang coz he always plays well in rally even when he serves less good...