Got some unofficial info on the injury - he twisted his ankle during the match and didn't want to risk making it worse. Nothing serious, it appears. After a week off, he'll be ready to go and try to qualify for some challengers next (according to a poster there).
I think that I've seen the point when it happened, as a remember a moment where he just disappeared from the screen, being stretched wide on the deuce court and was kind of checking his leg and dusting himself off after that. The fixed POV on the live cam kind of sucked, didn't see the possible fall itself, so that's that.
Link (in serbian but google translate works)