I only noticed it after saving it on my desktop. But I love him too much to let some sweaty armpits bother me. I am not that shallow.
I'm not so sure it is sweat because, if you compare all the pics, Andy must have been producing the same huge amount of sweat in each match, which seems odd particularly as Wednesday's conditions were damp and chilly, and the continuation of the Mahut match lasted around 45 minutes while he saw Matosevic off pretty quickly as well, which is nothing in the kind of temps they've been having at Queens, i.e. 62F at best (and this is the man who trains in 80F+ in Miami which has high humidity as well), so I think it's just a continuation of the darker blue colour on his sleeves.