Haven't seen any of his matches so far so I can't really say much about Zverev's level of play. Making it this far without droppint a set is very impressive but I don't think he's faced anyone who was a real challenge so far (Rublev is obviously a favourable matchup for him). I think, in the end, this may just come down to mental toughness. Zverev's is extremely suspect under pressure whereas Stan is normally pretty clutch (and certainly was against Medvedev). Stan in 4.