Henry - there are a good number of serve-volleyers that Kunitsyn has lost to, though (even Grandpa got him, at Queen's recently), and he isn't playing at the same level he was only a matter of months ago. But Zverev has hardly been setting the world alight, recently, either...toughie.
I should've added "on hardcourts" because he's hopeless on grass and I wouldn't read much into his results on it. Guccione is the only s/v I can defeat I can see this year on hardcourts and Guccione is more of an all serve player than a traditional s/v.
But his form looks dodgy (won 4 games v Stoppini) and I'm leaving it alone.